NARRATOR: Underneath the streets of London, an army of more than 10,000 engineers is building a brand new subterranean railway.

Okay!

MAN: We've done the maths, we've checked the maths, and we've checked them a third time.

NARRATOR: Costing almost $23 billion, it's the biggest construction project in Europe.

I've been doing civil engineering for 35 or so years.

Even I can't appreciate the scale until I come down here.

NARRATOR: Workers are digging 26 miles of tunnels and constructing ten vast new stations all under a tight deadline.

WOMAN: You need to get out of the way here, because trains are gonna start coming through.

NARRATOR: They're over halfway through a ten-year project.

That's ready to start pumping!

NARRATOR: In a constant battle to keep the city moving.

MAN: Constructing Crossrail is like undertaking open-heart surgery on a patient whilst that patient is awake.

NARRATOR: The oldest rapid transit system in the world, the famed London Underground is growing even bigger: 75 new miles of railway in the midst of a city that never stops.

Inside the "Super Tunnel," right now on NOVA.

Major funding for NOVA is provided by the following... people.

The key to keeping everyone moving: the Tube.

The London Underground was the world's first metro system, a vast subterranean rail network handling over a billion journeys a year.

It's made up of ten separate railway lines that snake right underneath the city.

A network of escalators and walkways allow commuters to disembark from one line and board another to travel in any direction across the capital.

Victorian engineers opened the first line here in 1863 at an estimated cost of a million pounds.

It was built to solve what was, even then, London's massive traffic problem.

To build it, engineers closed streets for years, cutting huge trenches, laying track, then covering the holes back up again.

Today, the traffic in London has gotten even worse.

(horn honking) CABBIE: You just gotta keep your cool and just go with the flow, not get too stressed out about it all, because you'll just drive yourself mad.

(horn honking) NARRATOR: This makes the Tube even more important-- and crowded.

The 150-year-old network struggles to cope with peak demand.

So today, engineers are building a brand-new underground railway line to help relieve the strain.

It will stretch right across the city, from the east to the west.

It's called Crossrail.

It will run overground from Heathrow Airport in the west in a new tunnel under Central London connecting into existing metro stations.

It will link shopping and theater districts in the West End to the new financial district of Canary Wharf and terminate 14 miles east of the city center.

It must link seamlessly to the rest of the Tube and be ready for passengers in 2018.

If all goes well, Heathrow, Europe's busiest airport, will be just 28 minutes from London's West End, a journey that currently takes almost an hour on the Tube.

An idea born in 1974, Crossrail is now the biggest engineering project in Europe.

Building Crossrail in the middle of nowhere would be a big enough technical challenge, but to do that right in the center of London with all of the neighbors above us and around us makes it more complex still.

NARRATOR: Today, there are 40 worksites spread out across London.

Some are little more than shafts allowing access to the new train tunnels growing underground.

Others are giant holes puncturing the landscape, forming the outlines of ten new stations.

The sites in Central London are hemmed in between office buildings, busy shops, and heavily congested roads.

And that's just at street level.

Things get even tighter underground.

This is Oxford Street, the heart of the London shopping district.

It's the busiest commercial street in Europe.

Home to flagship stores including the legendary Selfridges department store, 200 million people visit Oxford Street each year.

Few shoppers here would ever suspect that just 80 feet below, a 500-member team of engineers and builders is chewing through the earth.

Tunnel construction manager Steve Parker keeps the project moving, hopefully without disrupting life on the street above.

PARKER: In the future, I want to be taking my family in this tunnel and say, "Look, I worked on this."

I think many tunnelers like to think of themselves as kind of the unsung heroes because it's all underground.

NARRATOR: Steve has been building tunnels for over 25 years.

He started out as a construction worker, but now oversees the operation of two giant tunnel boring machines, called TBMs.

In all, there are eight of these dirt-eating monsters.

Each one is 490 feet long and weighs 1,000 tons.

The machines have been boring away for just over a year, but now Steve's crew is about to face its toughest challenge yet.

The brand new tunnel and ten new Crossrail stations must connect into the existing London Underground network.

This means the tunnel boring machine must drive dangerously close to tunnels and other structures.

But only once will they pass this close.

Engineers prepare to inch one of the tunneling machines through the tightest point of the entire route, known as "The Eye of the Needle."

At the east end of Oxford Street lies London Underground's Tottenham Court Road Station.

Crossrail's new tunnel and station here will link into two existing Tube lines called the Northern Line and Central Line, creating a Central London super hub.

But digging tunnels here is not easy.

Pipes, cables, and sewers crowd the ground.

The Tube's busy Northern Line platforms and two escalators make the earth extremely crowded.

The only option for Steve's team is to drive their tunneling machine through the tightest of gaps, 33 inches above the live, running Northern Line and 14 inches below the escalators.

It's the closest any Crossrail tunnel will come to the critical infrastructure that keeps London ticking.

At the controls for the tightest drive of them all is engineer Ed Batty.

We've been in tricky spots before, but nothing where we've had something below us and above us in such a close proximity, so yeah, it's a first for me.

My first job on a TBM, and this is one year and one month I've been down here.

The first six months was a learning curve, and now I know what the crack is, basically.

NARRATOR: Steve and the team have a crucial meeting with London Underground to coordinate their next step: how to avoid interrupting Tube services.

MAN: C'mon!

NARRATOR: The tunnelers can't cause any interruption during the 48 hours it will take their machine to pass through the Eye of the Needle.

The tunnel boring machine is passing directly over a platform tunnel, so our customers will be able to see the impact of the tunnel boring machine passing by.

For example, you could have tiles falling off.

If we had customers on the platform who started seeing a lot of fluid come in, they might cause a panic.

If the worst comes to the worst, we might have to evacuate the station.

The only thing we've got to bottom out is what surveillance regime we're going to have in place.

NARRATOR: The team must keep a close eye on the Tube platform while the machine passes overhead.

They will have just seconds to halt the operation should a crack appear.

PEARCE: I suppose the real excitement, if you like, or the adrenaline will start if there is an incident.

NARRATOR: If all goes well here, trains and people passing through this station won't miss a beat.

But if any of the other construction sites fall behind schedule, the entire $23 billion project could be derailed.

One site that could easily become a problem is the new Crossrail station at Canary Wharf, London's financial hub.

Here, workers are building an utterly unique structure.

Since the 1980s, Canary Wharf has been transformed from a derelict post-industrial wasteland into the Wall Street of London, home to many of Europe's tallest buildings.

Over 100,000 people now work in this booming financial jungle, so the area needs better transportation links to help get them all there.

At 1,050 feet long, Crossrail's Canary Wharf Station is vast, as long as three-and-a-half football fields.

But because space around the docks is at a premium, they had to build it underwater.

To do so, they sank an 840-foot-long watertight concrete box to form the station's walls.

Concrete pillars anchor it into the dock bed.

They drained out more than 40 Olympic swimming pools of water, then dug down to create four additional floors below water level and built two floors above.

To transform this concrete box into a station, first they must build the platforms, then install the escalators.

But that's a walk in the park compared to assembling its intricate roof from 1,500 timber beams to house a public garden open 24 hours a day.

This ambitious thousand-ton canopy will be built from specially engineered timber beams manufactured in Austria, joined together using 860 steel connectors.

The wooden frame will be covered by air-filled plastic panels, enclosing the rooftop garden.

Overseeing the German construction workers who have six months to assemble this giant 3-D puzzle... NARRATOR: ...is Phil Duffy.

One or two of our lads don't speak much English, so it helps a little bit to be able to speak a small bit.

(speaking German) Okay, Prem, up on one on the hoist, up on one.

NARRATOR: Phil's first milestone: assemble the first arch of the canopy.

If we don't have these in the exact right position, the timber elements won't fit.

Tiny bit down?

DUFFY: Everything has to be within the millimeter.

This is like the keystone.

The big crane will lift this in, and hopefully then it will slot in to the exact right position.

If this aligns up, the whole structure will follow through, so this will be one of the most critical lifts of the whole thing.

Have you got a knife on that little Swiss thing of yours?

It's for nails, but... NARRATOR: The first arch needs a keystone, a two-ton timber unit that will hold the structure together.

RAY PALLETT: We're just about to put the center section in place using the tower crane.

We'll get everybody else out of the way, and hopefully it will go well.

As you slew round, mate, that should stay same orientation.

Heads up, heads up!

Nice and steady, inches at a time, please, mate, nice and steady, come down on your wire.

NARRATOR: The keystone is in position, but it's not a perfect fit.

Just trying to locate the bolts on each corner.

It's literally millimeters out.

If we have an issue here at one of these timber structures here, it means it exaggerates as you go along the building and our connections won't go in correctly.

For alignment, they have to be perfect, within maybe five mill.

NARRATOR: So they apply a bit of brute force.

PALLETT: Hit it properly, Robin!

(laughs) You can't beat the sledgehammer at the end of the day.

(laughing) Yeah, that's it.

Fairly chuffed, yeah.

Oh yeah, I think I should have been a photographer.

What do you think?

If you stand back and look at it, you can see the whole arch from one side to the other side now, which is perfect.

This is a massive structure, like, and when it goes to plan, you can't be happier.

Looking more than good, looking brilliant.

NARRATOR: First milestone complete.

Okay!

NARRATOR: They are on schedule for now, but there are 1,300 pieces of the puzzle to go.

And the station is only five short miles from Tottenham Court Road and London's shopping district.

Already a large station, it's growing further, adding a network of underground walkways and escalators to connect it to Crossrail's new tunnel and platform.

When completed, it will become a greatly expanded gateway to the shops above.

Just off the main thoroughfare is the 335-year-old oasis of Soho Square.

Crossrail is building a new station directly underneath our feet here.

Our tunnel boring machine is directly under this building opposite us there.

NARRATOR: Advancing on average 72 feet a day, Crossrail's vast tunnel boring machine is closing in on the Eye of the Needle.

The Tunnel Boring Machine, or TBM, has sharp cutters in a huge rotating wheel that scrape at the earth like a drill.

Behind this cutter head is an enclosed steel jacket that holds the earth at bay and creates a safe area for the tunnelers.

Eight pre-cast segments make up each ring, and workers secure them with bolts.

Once a ring is complete, hydraulic rams push the machine further forward into the ground.

Every five feet they advance creates space to build another ring.

In perfect conditions, this digging demon can build up to 45 rings a day, leaving a water-tight, tube-shaped train tunnel in its wake.

ALDER: We've got to pass under a couple of buildings before it gets to the Eye of the Needle, but before we get there, obviously we've got to protect the buildings.

We don't want to put the table tennis table out of level.

It's all part of keeping London moving.

NARRATOR: Digging directly under a city is a delicate operation.

The loose ground around freshly-dug tunnels could settle unevenly, potentially tilting and damaging buildings.

So Crossrail's engineers use a network of lasers and targets, capable of registering even a millimeter of movement in any of the buildings.

ALDER: If you look closely on the buildings, you can see lines of these prisms that are all across the facades.

On the far corner there on the brackets away from the building, you can see an automatic station up on the end there.

It will know where these prisms are supposed to be, it'll turn the instrument to see where it last read the prism from, and then it will take the shot that will give it the exact location of the prism.

You'll see it rotating round now, working its way around, and then sending all that data back to the control room so we know where all these prisms are in real time.

NARRATOR: Data from thousands of targets installed across Central London flows back to Tunnel Control.

Here, Simon Leavy specializes in the analysis of this data, picking up the slightest change in ground level.

If the ground moves either up or down, we can tell from these graphs.

The nodes on the points are blue, so that means they're not in any trigger area, but if they go to a green, it's a green alert, amber and red.

NARRATOR: Robot trackers keep an eye on some of the oldest buildings in London 24 hours a day.

Among them is one of the most celebrated structures next to Oxford Street, in historic Soho Square: the mid-18th century charity and chapel House of St. Barnabas.

MAN: This is the plasterwork installed in 1754.

It's a classic piece of Rococo work.

The main hall and the Silk Room next door together constitute the last complete set of Rococo plasterwork in London.

NARRATOR: This building is under constant surveillance.

A steel frame stands ready to catch the 260-year-old staircase, should it collapse.

The House of St. Barnabas is bristling with instruments.

But despite steps taken to support its stairway and protect its pillars, there's still a risk.

As workers dig beneath Soho Square... ...the excavations are disturbing the ground.

LEAVY: In Soho Square now, we have some amber triggers on the leveling points.

It's not to do with the TBM; it's the new ticket hall.

They're actually excavating fairly deep in that area.

NARRATOR: Sensors on the House of St. Barnabas have triggered alerts.

It looks like cracks in the rare plasterwork are getting worse.

The corner behind me has been gently moving towards the square.

And we are watching the cracks that are forming in the plasterwork.

Now, we don't want to be panicky about this, but you have to keep an eye on what's going on.

The main thing is to try to ensure that it all stays up there.

NARRATOR: Tunnels beneath Soho put these historic buildings at risk, but the stakes are even higher to the east, where another team is digging below the River Thames at Custom House.

The train tunnels for Crossrail need to pass under the Royal Docks here.

In their Victorian heyday, these were some of the largest docks in the world.

And today, they include London's largest exhibition space: The Excel Center.

The new Crossrail station will make it easier and faster to get here, as well as to London City Airport.

Much of the old Victorian infrastructure remains in place today, including the Connaught Tunnel that runs underneath the water here.

Linda Miller, an American engineer, heads a team that is attempting to rebuild this old tunnel to make it suitable for modern high-speed trains.

The mission for the Connaught Tunnel team is to turn a 135-year-old beautiful piece of Victorian architecture to a state-of-the-art modern tunnel.

NARRATOR: The existing tunnel is too small for Crossrail's trains to squeeze through.

MILLER: Well, I've been on some very exciting jobs.

I've been lucky enough to build a new space launch complex at Cape Canaveral, Florida, and do tunnels in other beautiful cities, but I reckon this is my favorite job yet.

I love the idea that we're bringing beautiful old heritage railroad back to life.

NARRATOR: The Connaught Tunnel was built in 1878.

Steam trains once ran through here, shuttling passengers back and forth to a ferry terminal.

MILLER: You may just see the old coke deposits and memories of the steam trains left above there, but actually, what I see is a tunnel that's in cracking good condition, fantastically well built, you know, really built to last.

NARRATOR: Dismantling and rebuilding this robust underwater tunnel will require a complete rethinking of the structure.

For half of its length, the Connaught tunnel is a single tunnel with two sets of tracks.

But not in the center under the docks, where it splits into two.

Linda's team must completely rebuild this section, creating a single taller, deeper, and wider tunnel big enough for two Crossrail trains.

MILLER: The first job then is to start to deepen this tunnel, and you can see that's just what we've done, cutting away and uncovering bricks that haven't seen the light in 130 years.

NARRATOR: Linda's team must also remove the steel rings that Victorian engineers used to form arches, reinforcing the tunnel roof here.

But a survey has revealed that there is a problem with this plan.

MILLER: It was always assumed that we could cut these cast steel rings out and replace them with rings that were slightly larger and that that would all be fine because we had a really good level of cover above the crown of this old tunnel.

So it was shock and dismay after we had our first divers clear away quite a lot of silt that was at the bottom of the docks and do a proper survey and find that actually, we have no cover at all.

NARRATOR: That means that the bottom of the dock is perilously close to the steel rings.

The current plan to enlarge the tunnel could lead to deadly consequences.

MILLER: The word we were worried about is, "Oh, my gosh, as we try and cut these rings off "of the crown of this roof and that much water is above us, catastrophic inundation," or the sluicing in, uncontrolled sluicing in of the Royal Docks into this tunnel became quite the real... well, terror, really.

NARRATOR: With little or no soil separating the tunnel from the water above, removing the steel rings could cause a catastrophic breach.

The only way to expand the tunnel safely is to seal off the passage with giant steel barriers, drain the water, and rebuild the tunnel top down from inside this dry workspace.

But closing off the waterway here could be a problem.

It's the only way river traffic can pass to and from the city's largest exhibition space, Excel, home to the annual London Boat Show.

They can't start the process until after the Boat Show, when the road bridge can be opened to allow the ships out.

And then they have a narrow time window of just six months to complete the work.

If the passage isn't open by then, naval ships arriving for the annual Defense Show won't be able to reach the hall.

Time is also against another team... ...three miles west along the River Thames in the busy new financial district of Canary Wharf, where Crossrail's ambitious timber roof is slowly taking shape.

But deep below the roof, engineers are preparing for a key milestone.

One of the eight tunnel boring machines is now nearing its first target.

It must break through into Crossrail's vast new Canary Wharf Station, connecting the tunnel with the platform space.

We're at the minus six level of the Canary Wharf box, and we're just waiting on our first TBM to pop its head through for our first breakthrough.

This is our first breakthrough on an existing structure, so for us, this is all about making sure that the machine is where it should be.

NARRATOR: Canary Wharf Station is a giant six-level box with a garden, shops, and restaurants on the upper floors, and the platforms deep below ground.

The team here must drive its tunnel boring machine through the station's concrete walls, hitting a specially designed target.

Crossrail's first breakthrough is just a few inches away.

(applause) (cheering) NARRATOR: Everyone is jubilant as the machine hits its target spot on.

Canary Wharf Station is now connected to the new tunnel... MAN: One, two, three!

(cheering) NARRATOR: ...putting the project one step closer to joining up with the hub at Tottenham Court Road.

But in neighboring Soho Square, the robot laser trackers have detected a dangerous shift in the ground, threatening the House of St. Barnabas and putting its rare Rococo decor in peril.

Engineers must stop the earth from sinking before it gets any worse.

Before tunneling began, they dug 22 shafts around Central London, part of a subterranean system to protect historic buildings.

Four of these shafts are in Soho Square.

A spider's web of thin tubes stretches out from each shaft.

Each tube has holes every three feet.

Engineers send a narrow device called a packer to the spot where the ground is settling.

The packer precision-injects grout to fill up any voids, lifting the earth back to its original position, protecting plasterwork, preventing further cracks, and keeping buildings safe.

MAN: TAM number 46 then, yeah?

NARRATOR: This shaft in the southeast corner of Soho Square is where grouters Lloyd and Tony work to shore up the buildings.

LLOYD: This is what we call a packer.

This rubber part here will inflate.

That will form a seal to prevent any grout coming back out.

90 meters is a long way to push the packer, but you take the rough with the smooth, I think.

2-6-1-5!

Okay, 2-6-1-5.

NARRATOR: Lloyd and Tony spend up to 12 hours a day down this shaft, packing holes under Soho Square.

LLOYD: Okay, that's depth.

inflate the packer.

That's ready to start pumping!

Yeah, pumping now.

I've been on the job for about 16 months.

Tony's been with us for about six months.

We generally tend to stick together as a team as well, you know?

Yeah, we do come...

I mean, not too close, obviously.

You don't want to get too close in a place like this.

You get used to talking to yourself, but apart from that, it's all right.

13-3-0-7-5!

Okay, up on the reel!

Whoa!

Welcome to the House of St. Barnabas periodic meeting.

NARRATOR: In order to keep these historic buildings safe, the Crossrail team meets once a month to analyze the laser leveling data.

WOMAN: And then the summary sheet won't have any triggers unless we get real movements.

NARRATOR: They'll keep a close eye on the House of St. Barnabas and its Rococo plasterwork.

SCOTT: The charming lady here has survived to keep us entertained in the 21st century and I hope for many more.

NARRATOR: Finding a way to protect historic buildings above ground has been tough, but a few miles away, engineer Linda Miller struggles with a potentially disastrous situation.

She needs to expand a 19th-century tunnel running beneath the docks next to the River Thames.

But the lack of headroom has created a huge problem.

Now she's racing to find a way to do the job without causing a flood.

Faced with the reality that she will first need to drain the channel... MILLER: Thank you very much.

How exciting!

NARRATOR: ...Linda decides to read up on the tunnel's history.

Now, this was the later stamp...

The original drawings, 135-year-old drawings there.

DAVID WILDE: We've been presented with the same problems as the original construction, basically.

A lot of it's to do with water and how you actually build something with all the water around it.

NARRATOR: She discovers that her plan actually mirrors the techniques used by the engineers who originally built it.

MILLER: "Existing dam."

See, that's history repeating itself there.

Instead of tunneling underneath the ground, they've actually excavated around the profile and then installed the tunnel in there.

So they've built it from the top downwards, effectively.

MILLER: Looking at the two twin-walled cofferdams standing there with "1872," "1874" written in the corner of the drawings, I think it was meant to be.

NARRATOR: Finally, the London Boat Show is over.

As the last luxury yachts cruise out of the docks, a narrow window of opportunity opens for Linda's team.

They can now close the passage directly above the tunnel and drain it.

But they still don't have the luxury of time.

They will have to reopen it again in time for navy ships to get to the Defense Show in just six months.

MILLER: We'll try and quickly get in here, do open heart surgery on this tunnel from the top, rebuild it into a larger tunnel, and get out of here by the time the Defense Show comes.

It's hard work.

NARRATOR: The last of the water is drained from the dock, exposing the 137-year-old roof of the Connaught Tunnel.

PAUL OSBORNE: We've got lots of workfronts going on, so we're working in the tunnel, working in the dock, we're all a team, basically, working together trying to achieve one goal.

NARRATOR: The team must complete a laundry list of challenges in a very short time.

First task: remove the steel rings lining the twin tunnel section.

What you're able to see quite clearly here, now that the docks are empty, you can see the cast steel barrel.

The Crossrail tunnel is going to be wider and rectangular and fit its haunches within the old tunnel.

NARRATOR: Carefully, the team removes the rings without triggering a collapse.

But as they cut the steel away in this corner, they reveal a completely unexpected brick arch behind.

That's a problem.

NARRATOR: The new tunnel is supposed to fit inside this arch, but there's not enough clearance for the new tunnel to fit in.

That's got to be removed.

NARRATOR: They can't just remove the brick arch behind the steel rings because it could be supporting the docks above.

MILLER: You're not going to be able to take a section out of this and still be able to hold on to your arching effect.

Pretty unlikely.

Unlikely.

Is it the same on the other side?

Yeah, but we don't know how far.

All the rings they've taken out so far, we've got the reduced dimension.

All right.

Yeah, this is yet another time when this tunnel shows us new mysteries.

This job actually started construction a year ahead of when everyone said that it needed to, and it was because a predecessor of mine said, "It's going to be a bucket of spiders."

And oh, my goodness, have we used every bit of that, and now we're staring at the end date that we never thought that we would need to be worried about.

NARRATOR: Linda and the team are already facing a very tight deadline.

They can't afford this new delay.

And they quickly discover that the brickwork is unusually tough.

MILLER: The mortar between them is 100% full.

There's no gaps here at all.

It's a fantastic, fantastic job.

And then the 135 years of earth pressure and water against it has sealed it up to where it's behaving more like stainless steel than it is brick and mortar.

NARRATOR: The strong mortar is now causing the latest problem for the team.

They can't get the bricks out quickly enough.

OSBOURNE: Just over a third of the way to go.

It's quite slow going, this brickwork.

You know I've seen brickwork like this taken down, and all you normally need to do is have a couple of stabs at the mortar and a whole brick layer goes off; couple more stabs at the mortar, the next layer goes off.

I know the men are taking a short break here now, but hand-breaking out this 130-year-old brick is just, well, hard work.

It's going to take a while.

I know they're working it night and day, I know they've got extra crews in, but there's not much time.

Here we are in the last throes of the last couple of weeks before we put the water back in.

We couldn't be throwing more into it than this.

NARRATOR: Linda's team works around the clock to remove the protruding arch inside the Connaught Tunnel and build a new roof of steel and concrete before they must re-flood the channel in the next 24 hours.

The final push pays off as water streams back into the area.

Linda and the team have completed the tunnel just in the nick of time.

WOMAN: I can't believe you can see all the way through!

No one's ever gonna have been able to see that all the way through.

MILLER: Doesn't it look fantastic?

I know, it's mad!

I can't believe it's finally done.

Feels so good.

Like, we're directly below the docks right now and you wouldn't know.

I know, below meters of water there above us.

My gosh, we were working like dogs, weren't we, 24/7?

It couldn't have been closer, it couldn't have been closer.

WOMAN: We've done it.

NARRATOR: With the tunnel complete, the dock can reopen, allowing the military ships to pass through.

The Excel's Defense Show can go ahead on schedule.

This is the new Connaught Tunnel for the next 120 years.

So you need to get out of the way here because trains are gonna start coming through.

NARRATOR: While this old tunnel has been given a new lease of life, at Tottenham Court Road, a brand new tunnel will soon squeeze through the congested earth here.

The thousand-ton tunnel-building monster is finally entering the Eye of the Needle.

Welcome, everyone, to Sunday morning the 8th.

NARRATOR: This is the day Steve's team has been working towards.

That's where we are at the moment, just touching the side of Charing Cross Road.

PARKER: We're under the site of the old Astoria Theatre, aren't we, Willie?

ARCHIBALD: Yep.

NARRATOR: Today, the tunnel-boring machine will reach the narrowest point of its route across London.

MAN: So the crossing starts on back shift this afternoon, and I think we're going to be there, yeah.

Today is the critical day.

It's the start of passing over the Northern Line, and so this is the critical point, the culmination of a lot of work over the last couple of months.

So people getting off the train in the next hour or so will not realize that above their head is a 900-ton, 7.1 meter-diameter tunneling machine.

NARRATOR: It's vital the close encounter doesn't cause water or concrete to fall onto the Tube platform below, which could spark panic.

General comment is to say, "Be aware of proximity of LU assets."

MAN: I mean, I would like Ed to keep an eye on the belt.

Yeah, just extra vigilance.

So over the next 20 rings, we're directly above the Northern Line platform.

The Eye of the Needle.

We're just about to go through it.

(phone ringing) Hello?

Hello, Ed.

Hi, Steve, how you doing?

We've got one more ring to go before the cutter head gets in line with the angle of the Northbound Line.

Tim Morrison of LU said he was down this morning, he was there, and he said he could hear the TBM and hear the miners.

(faint drilling noises) BATTY: We're that close, so you can actually hear what we're doing here.

PARKER: Yes, he could hear, but that with no trains running.

Our one concern is that there are cracks within London clay, some of the water could ease out and find the simplest path of travel, which could be the big platform tunnel.

BATTY: The TBM cutter head is now directly above the Northern Line northbound station platform, so about 850 millimeters below my feet is the crown of their tunnel, so not a big distance at all.

You see where there's a blockwork wall here, just behind the tiled edge?

That is pretty much the center line where the tunneling machine is actually crossing this structure.

NARRATOR: With the tunneling machine less than three feet above, the pressure on the Northern Line platform tunnel will be immense.

The team is worried about lubricating fluid pumped into the earth ahead of the machine being forced through the wall into the platform.

MORRISON: Sam is one of the guys who's been based down on the platform.

He's specifically looking for any fluid ingress from the tunneling machine, because that is something that we are concerned as a possibility.

SAM: The tunneling machine at the moment is quite literally above the tunnel crown.

There is that apprehension because there is a small risk that we could see some ingress, and so I guess that makes it more exciting.

NARRATOR: With the tunneling machine now threading through the Eye of the Needle, the team must continue their vigil throughout the night.

As London sleeps... (whirring) ...the 490-foot-long earth-eating giant continues its relentless drive.

With every passing moment, the cutter head of the machine draws ever closer to the Northern Line and its escalators.

Willie, got an update where we are?

Yep, they're building 3024 just now.

NARRATOR: Finally, the team and machine squeak through the tightest spot with only inches to spare.

Yep, okay!

NARRATOR: A complete success-- no terrified passengers, no evacuated platforms.

It's time to breathe again.

PARKER: We've passed over two platform tunnels with a 900-ton tunneling machine.

That's even a first, I think.

MORRISON: Yes, it is, yeah.

Cheers.

BATTY: The lads have been working down here really hard, and so have the guys up in the control room.

A big relief.

I'm chuffed that we've done it so well and we've had such good results.

We've done it, yeah, got through the tricky spot.

PARKER: The trains have kept running.

Passengers haven't known that we've been there.

It's been a great achievement, and I'm glad to be part of that team.

NARRATOR: The team leaves a perfectly formed Tube tunnel in their wake, gradually closing the distance to the new financial heart in East London.

Here at Crossrail's Canary Wharf Station, Phil's team is locking the last beams into place.

DUFFY: Everything's going perfect, thank God.

Everything is good.

It's the biggest project that we've done.

Something to be proud of, I suppose.

NARRATOR: Final piece of the puzzle: 780 inflatable panels known as cushions that will fill the gaps and cover most of the rooftop garden.

Installing them requires a head for heights, so they've assembled a crew of builders with a special talent.

MAN: They're rock climbers and mountain climbers, so they're used to heights, and yes, a very skilled trade.

Basically, they're just unwrapping the cushion now.

It comes all folded up, they unfold it.

They have to put aluminum sections which run down the edge.

Then they'll connect it into the system, connect the air pipes, and blow it up.

Sounds simple; little bit more difficult.

NARRATOR: Fitting the first cushion on the lower outside edge will be a real test.

Below is a 65-foot drop.

Because we're installing the cushion almost on the vertical face, there is no area for a safety net.

This is the...

The most susceptible point of the installation is when it's opened.

If the wind catches it, then that's the worst-case scenario.

Once it's got the rails on it and it's attached, it's secure.

They're now fitting the air inlet, so any moment now, you'll see the cushion inflating.

It is actually starting to inflate now.

One done, about 750 to go.

I'm a happy bunny.

MAN: It's growing, you know?

It's getting more beautiful every day.

NARRATOR: 130 feet below the roof, the station's 790-foot-long platforms are being fitted out.

And the final timber slots into place on the roof.

All our timbers are fitted, everything is done now.

Someone else's problem now.

I'm out of here, back to Ireland.

(humming cheerful tune) NARRATOR: So far, Crossrail's 10,000 employees have put in a hundred million working hours... (cheering) ...poured over 88 million cubic feet of concrete, and laid over 200,000 tunnel segments.

In 2018, the ticket halls will open and the escalators start running.

Before then, the team must lay the tracks and build the trains.

London's population is set to pass nine million people in 2018, and they will have a new $23 billion rail line to keep them on the move.

They come from below.

Media Access Group at WGBH access.wgbh.org This NOVA program is available on DVD and Blu-ray.

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