(engine buzzing) (host) In spring 1943... (gunshots) ...133 young men set out on the most daring and ingenious bombing raid in history to destroy the great dams of Germany using bombs that bounce.

Look at the way it's skipping across the surface of the water there.

If successful, they'll deal a hammer blow to the Nazi war effort.

(dramatic music) They're led by this man, Guy Gibson.

He's only 24, but he's already one of the most famous and highly decorated pilots in the world.

I'm Dan Snow, and over three programs, I'm charting the nail-biting countdown to the mission.

In less than two months, 6-1-7 will be the most famous squadron number in RAF history.

This time, we follow Guy Gibson as he's given just 60 days to form a crack squadron and secretly train them to fly their bombers on the edge of what's possible.

This is really low.

While maverick inventor Barnes Wallis struggles to make his revolutionary weapon work.

Barnes Wallis groans, "Oh, my God."

This is the story of the single mission that can change the course of the Second World War.

♪ These are the Dambusters.

♪ (mellow music) ♪ St. Vincent's Hall, Lincolnshire, headquarters of Bomber Command's 5 Group.

♪ The top-secret nerve center from where, night after night, the British are orchestrating the aerial bombardment of German cities.

♪ It's early on Thursday, the 18th of March, and Wing Commander Guy Gibson is arriving here at HQ for the fourth morning in a row.

♪ Guy Gibson has no idea why he's here at headquarters.

He's not happy about it.

It doesn't suit him.

It's full of cold, efficient-looking men marching around these corridors with red folders under their arms.

He is a front-line bomber pilot, far more used to the adrenaline-fueled life of a wartime squadron.

(soft music) In fact, Gibson shouldn't be here at all.

He should be on his way to Cornwall with his wife for a much-needed spot of leave.

(engine buzzing) He's just completed his third tour and has now flown over 150 dangerous missions.

(gunshots) He's already defied the odds.

Very few RAF aircrew survive two tours.

♪ After days hanging around at HQ, Gibson is finally called to wait here outside the office of Air Vice Marshall Sir Ralph Cochrane, the second most senior officer in Bomber Command.

♪ Gibson walks in here, into what was Cochrane's office.

Cochrane's in here and congratulates him on his latest gallantry award.

Then he says, "How do you feel about one more flight?"

Well, Gibson knew full well that he's completed more than his designated number of flights.

He's spent years dealing with the anxiety caused of night bombing raids, seeing his friends die around him, other aircraft crashing to the ground in fireballs.

He's aware that his own survival is something of a miracle.

So not surprisingly, he's less than enthusiastic about this one more flight.

(solemn music) (man) When Guy Gibson walked into Ralph Cochrane's office and was asked if he'd do one more mission, he was already a legend.

Now, some so-called war heroes, you can be a bit iffy about whether they were the real thing.

Gibson was the real thing, incredibly courageous.

A lot of the men who served under him hated his guts, but incredibly brave, incredibly determined.

♪ (woman) Gibson was really suffering from burnout.

He was struggling with this mental strain of having to carry out so many operations, so many sorties.

And finally being told, "Here's one more for you."

(soft music) (Dan) We know that standing in this office, Gibson's head was full of concerns and emotion.

And yet, all he actually said was, "What kind of trip, sir?"

Cochrane replied, "A pretty important one, perhaps one of the most devastating of all time.

I can't tell you any more now, but are you keen to do it?"

Gibson remarkably thought he was being asked to go on a raid that very night.

And so despite all his reservations, he says he was pretty sure he was up for it, but he hadn't brought any of his flying kit.

♪ Cochrane tells him this is no ordinary raid.

It's possibly the most important mission of the war so far.

And Gibson will need to form and train a special squadron to carry it out.

Cochrane impresses upon Gibson the sense of urgency.

This highly secret, very dangerous raid needs to take place in under two months' time.

Gibson asks about training or about the target.

And Cochrane says, "I can't tell you any more than that at the moment, I'm afraid," so that was that.

♪ (dramatic music) ♪ Just three days later, Guy Gibson arrives here at RAF Scampton.

It's one of the dozens of airfields that pepper the flatlands of Lincolnshire.

(engine buzzing) This is the epicenter of Britain's huge heavy bomber offensive.

♪ Putting this base at the cutting edge of Britain's entire war effort.

♪ (Max) Between June, 1940, when the British Army was kicked out of France and June, 1944, when it went back with the Americans, most of the British Army was training in Britain.

And for those four years, the bomber offensive was the most important thing Britain was doing to carry the war to Germany.

Churchill embarked on it when he could see no other way of defeating Germany than a bomber offensive.

♪ (Dan) RAF Scampton hasn't changed much since that Sunday afternoon Gibson arrived here in March, 1943.

♪ This was to be the home of the new secret squadron he'd been ordered to create from scratch.

♪ There hadn't even been time to issue it an official name, so for now, it was known as Squadron X.

So, Gibson has got to recruit 21 crack bomber crews into a squadron with no name and no aircraft for a terribly dangerous mission.

And he's got just a few days to do it.

The clock is ticking.

♪ (intense music) ♪ Guy Gibson, the young commander of the newly formed Squadron X, now has less than eight weeks to recruit and train 147 aircrew.

♪ He moves in here, the now derelict officers' mess at RAF Scampton.

It's also home to the bomber crews of 57 Squadron.

♪ The dining room would have been just behind here, and it's into this dining room that Guy Gibson comes for breakfast that morning in March, 1943.

And all around him are aircrew, aircrew of 57 Squadron, who were engaged in the nightly battle over occupied Europe, bombing enemy targets.

He obviously likes what he sees, because he manages to recruit five 57 Squadron aircrew into his brand-new squadron.

(solemn music) (Max) Gibson knew that he would need very special airmen.

They were going to have to be very brave, and they were going to have to be very skillful, because flying a heavy bomber-- people don't understand the physical demands that flying this huge airplane makes.

♪ In addition to the crews of 57 Squadron, Gibson phones up and recruits some of the best pilots in Bomber Command.

♪ But he needs a total of 21 pilots to make up his new squadron, and he can't hand-pick them all.

♪ So, the myth is that the entire squadron-- Squadron X as she is at the moment-- was made up of specialist, highly elite RAF pilots selected by Guy Gibson himself.

Not necessarily the case.

♪ (man) There were very veteran, very experienced pilots with lots of decorations.

But there were some very junior crews as well, some of which had only flown two or three missions.

♪ (Max) Squadron commanders didn't want to send him their best crews.

They attempted to send him one or two absolutely hopelessly duff crews whom they wanted to get rid of.

Also, as soon as people hear about a special operation, they're terrified.

Anybody who is in Bomber Command realizes that he's already in a business that is likely to kill him.

♪ And yet, here is somebody coming along now and said, "How would you like to come and join a special squadron that's going do something that nobody's ever done before?"

Well, they had a lot of trouble finding volunteers for that.

(soft music) ♪ (Dan) Only a couple of days after Gibson's arrival at Scampton, he's joined by the first of his recruits.

They include aircrew from Canada, New Zealand, and Australia who'll eventually make up around a third of the squadron.

♪ One of them is 23-year-old Les Munro, a tall sheep farmer's son from New Zealand with 17 missions under his belt.

Big Les had seen the request for volunteers pinned on his squadron notice board and volunteered with his crew.

♪ The day that Les Munro arrives on the base here, he meets the other recruits in the officers' mess for a beer.

He looks around, and he realizes immediately something's up.

He's never seen so many gallantry awards-- DSOs, DFCs--on the chests of the young men around him.

He realizes that if all of these star aircrew have been brought together, it must be a very, very challenging target.

♪ While the new recruits are gathering for a few beers here at the officers' mess, the RAF finally gives the new squadron its number: 617.

At the time, it was just the next available number.

But in less than two months, it will be the most famous squadron number in RAF history.

(solemn music) But while they may now have a number, no one on the new squadron knows what they've been brought together to do, not even the squadron commander, Guy Gibson.

♪ Six days after agreeing to lead the mission, Gibson is driven to a smart Surrey Golf Club requisitioned for the war.

All he knows is that he'll be meeting a scientist who's working on a secret project it's hoped will land a devastating blow on Nazi Germany.

♪ The scientist is Barnes Wallis, a senior designer for aircraft manufacturer Vickers.

♪ As Barnes Wallis prepares to meet Guy Gibson for the first time, the quietly spoken 55-year-old father of four must have wondered what kind of man this squadron commander would be.

Then in walks Guy Gibson, 24 years old, egotistical, heroic, bombastic, a charismatic leader of men.

They're from different worlds, one a man of action, the other a man of ideas, a blue-sky thinker.

But they strike up the most unlikely of friendships based on mutual respect and admiration.

(soft music) (man) We might think that this is a meeting of two opposites.

And to a certain extent, it is.

But war had a habit of bringing different people together for a common front.

Both of these individuals are sitting down and realizing that, in fact, they have a common objective, which is to win the war.

♪ (Dan) Barnes Wallis is one of Britain's most respected aircraft designers.

Now, he's also developing a precision weapon he believes can devastate Germany's industry.

♪ If the Nazis' production lines of tanks, planes, and armaments can be disrupted, the war can be brought to an early end.

♪ As Gibson and Wallis meet for the first time, Wallis looks around to make sure no one's listening and then says, "I'm very glad you've come.

I don't suppose you know what it's for?"

Gibson replies, "No idea, I'm afraid."

To which Wallis says, "What?

You mean they haven't told you the target?

That makes things awkward, very awkward."

(solemn music) ♪ The secrecy around the mission is so tight, Wallis is only allowed to reveal the target to a small number of people on an approved list.

Gibson isn't on it.

But Wallis can reveal the weapon 617 Squadron will be using.

In his office, he shows Gibson something extraordinary.

It's top-secret test footage of a revolutionary new bomb he's designing.

This is the actual footage that Barnes Wallis showed Guy Gibson of his prototype bomb.

There it goes, being dropped by a bomber, the bouncing bomb.

Look at the way it's skipping across the surface of the water there.

♪ This footage was filmed two months earlier.

(splashing) The successful test of Wallis's small-scale, prototype bouncing bomb helped convince the RAF top brass to back what some considered a madcap project.

Barnes Wallis had been working on this idea for years, and he was convinced this demonstrated that it was practical.

If you dropped the bomb at the right height, at the right speed, it would skip along the surface of the water before coming to a stop right next to the intended target.

On this occasion, it bounced around 20 times before coming to rest after travelling three-quarters of a mile.

♪ (man) Gibson must have been absolutely astounded when he saw the film.

The moment of revelation when it hits the water.

(splashing) And there's a plume of spray.

But then this weapon emerges from the spray and continues to bounce along the surface of the water.

What on Earth is all this about?

(soft music) (Dan) It's said Barnes Wallis first came up with the idea for his bouncing bomb as he skipped stones across the water with his family at Chesil Beach in Dorset.

He then began experimenting in his back garden using a tin bath, a catapult, and his children's marbles.

♪ (woman) Engineers often will start with something very small, like a marble, which doesn't seem to have any relation to bouncing a bomb.

But actually, you can learn a lot from a very small test.

♪ Just 11 months previously, Barnes Wallis was playing with marbles with his kids.

Now, he's showing Guy Gibson this film.

Trouble is, he can't tell Guy Gibson what the target is.

He just has to tell him that there are many targets within Germany that could be destroyed with the right amount of high explosives placed very accurately.

And this is the most reliable way of doing it.

The problem is, bouncing bombs require you to be flying at high speed at ultra-low level above water at night.

Nothing like it has ever been done before.

(tense music) Wallis asks Gibson if he thinks his squadron is capable of delivering a full-sized bouncing bomb to an as-yet-undisclosed target.

Gibson coolly replies, "It'll be difficult but worth a try."

(engine buzzing) He assumes they'll be attacking the fortified submarine pens on the French coast.

Or the great German battleship Tirpitz that's anchored in a heavily defended Norwegian fjord.

Ideal targets for a bouncing bomb, but near suicidal for the bomber crews.

♪ (solemn music) ♪ Back at Scampton, Gibson summons the crews he's so far recruited to a meeting.

Here in the huge aircraft hangar that's to be their new home.

The crews of 617 Squadron gather here in this main hanger at Scampton.

Guy Gibson jumps on the bonnet of a car and gives a speech.

He tells them the mission they've been selected for is dangerous.

The odds are heavily stacked against them.

If any of them wish to withdraw, they are free to do so.

Given that, there will be no stigma if they choose to walk away.

They will not be branded cowards or having a lack of moral fiber, as the RAF put it.

He waited.

Not a single person moved.

No one took up his offer.

♪ (dramatic music) The crews are about to begin intensive training.

They must learn to fly and navigate at extreme low levels.

They'll take to the air day and night until it's second nature.

♪ "If I tell you to fly through hangar doors," Gibson tells them, "then you will do so, even if your wingtips brush either side."

Discipline and total secrecy are paramount.

So all leave is cancelled.

But 21-year-old bomb-aimer Johnny Johnson, a farmworkers' son from Lincolnshire, is due to get married in a weeks' time in Devon.

Johnson is part of Pilot Joe McCarthy's crew, who's the only American in the squadron.

(Victoria) Joe McCarthy was massively respected, massively adored by his men.

He was this big, blond, strapping American who sort of used a few expletives and profanities that some of the perhaps more pursed-lipped Brits weren't quite used to.

(Dan) When he hears all leave is cancelled and Johnny Johnson will miss his wedding, Joe McCarthy is furious.

He marches his entire crew up the stairs and along this corridor to confront the squadron commander.

McCarthy knocks on the door of Gibson's office.

This was Gibson's office, and now, it's been restored just how it was back in the spring of 1943.

McCarthy storms in here, and he is in forceful form.

He points out that his crew have completed their tour.

They deserve some leave.

And what's more, his bomb-aimer is supposed to be getting married on the third of April, and he is getting married on the third of April.

Uncharacteristically, Gibson gives in, sort of.

He allows McCarthy's crew four days of leave.

(mellow music) Johnny Johnson travels to his fiancée Gwyn's hometown of Torquay.

♪ On the third of April, they're married in her local church with Gwyn wearing a dress she's borrowed because of clothes rationing.

Almost immediately, Johnson has to return to Scampton to fly an incredibly dangerous mission knowing Gwyn may never see him again.

♪ (solemn music) ♪ Eleven days after agreeing to lead a top-secret mission, Guy Gibson is once again ordered here to Bomber Command 5 Group headquarters in Grantham.

He now knows he'll be using Barnes Wallis's revolutionary bouncing bomb, but still has no idea what he'll be dropping it on.

Gibson returns here to the office of Air Vice Marshall Cochrane.

Now, once that door is safely closed, Cochrane points to three large packing crates on the floor in here and gives him a screwdriver.

Inside these crates, he tells Gibson, are targets that he is going to be expected to destroy.

Gibson opens the crates and feels a great wave of relief, because inside them, he doesn't find models of some huge battleship or U-boat pens.

Instead, each crate contains an intricately carved model of a landscape: hills surrounding a reservoir.

And at the head of each reservoir, a giant dam.

(dramatic music) Gibson's mission is to attack the great dams of Germany.

♪ The primary target is the Möhne Dam.

It's 130 feet high and nearly half a mile long.

When built in 1913, it was the biggest dam in Europe.

♪ Together with the other targets, the Sorpe and Eder Dams, it supplies much of the water and hydroelectric power needed in Germany's industrial heartland, the Ruhr Valley.

(Victoria) The big dams which were located within the Ruhr Valley and around the Ruhr Valley were so important to the German war machine because the Ruhr was considered the armory of the Reich.

It produced 70% of its raw steel, it produced pig iron, it produced all of these different aspects which were vital to keeping Nazi Germany in the war.

♪ (Hilary) There's a quote from Barnes Wallis: "150 tons of water is required to make one ton of steel."

And since steel is so important to the ultimate war effort, why not target the water?

♪ (David) Wallace knew that if those dams could be destroyed, this would cause a devastating blow to Hitler's war machine and production.

(soft music) (Dan) The great dams aren't just critical to Germany's war machine, they're a source of immense national pride for the Nazis, and the Allies have had them in their sights since the start of the war.

♪ In fact, in 1940, before Wallis came up with the idea for a bouncing bomb, he'd asked a group of scientists at the building research station in Hertfordshire to try and work out how the dams could be destroyed.

♪ During the particularly harsh winter of 1940 to '41, scientists built a perfect scale model of the Möhne Dam here on the edge of their research facility in Watford.

Now, apparently, it's still in these woods.

I'm going to go and find it.

(mellow music) ♪ Secrecy was vital, so only a handful of people knew about the model dam's existence.

I think the slope is going down.

I think there's a stream bed in there.

Everyone who worked on the project referred to it as Weir No.

1.

(birds chirping) ♪ Ah, here it is!

I love it!

Look at this!

(water running) (orchestral music) A perfect 1/50th scale model of a German dam across a stream in a wood on the outskirts of Watford.

♪ The scientists here carried out a series of tests to work out how much explosive would be needed to destroy the real Möhne Dam.

(explosion) The initial experiments showed that if you were able to drop a bomb in the water fairly near the dam but not right up against it, it would require 30,000 pounds of high explosives to make a breach like that.

That's the same weight as 10 cars.

(solemn music) But Barnes Wallis made a crucial discovery.

If a bomb detonated right up against the dam wall, much less explosive was needed.

♪ They did work out that 7,000 pounds of high explosives dropped 30 feet down right up against the concrete of the dam wall would breach it.

It was literally a massive breakthrough.

But how on Earth would Wallis be able to get a bomb that close to the wall of the dam?

♪ (orchestral music) ♪ By the end of March, the crews of 617 Squadron still have no idea what their target is.

♪ To maintain secrecy, they won't be told until just before the raid, which must take place in less than seven weeks' time.

(Max) They knew that the operation had to take place before the end of the May, because they had to attack when the reservoirs were full after the winter rains.

And if you attack them when the reservoirs are half-empty at summer levels, it's not going to do much harm.

♪ (Dan) With the May deadline fast approaching, the intense training required for the mission hasn't yet begun.

♪ But that's about to change.

♪ The new squadron can now start flying, because their new aircraft have finally arrived.

And back in 1943, there was only one aircraft capable of completing this mission.

And there it is, the Avro Lancaster.

(dramatic music) ♪ A magnificent plane.

This is one of only 17 Lancasters that remain of over 7,000 that were built during the war.

♪ The Lancaster might be a brilliant piece of British engineering, but it wasn't designed to carry a single massive bouncing bomb that now has the codename Upkeep.

♪ This remarkable plane needed heavy modification to carry an Upkeep bouncing bomb.

The bombs weighed four tons, three of which was high explosives, so they had to strip lots of heavy parts out.

They took that turret out, and they also removed a lot of the armor plating on here.

They also had to change this bomb bay.

They had to pull these off, because the Upkeep bomb was so big that it would sit half sticking out of the fuselage of the plane.

(solemn music) ♪ Inside, it was a tight fit for the seven crew members needed to operate the aircraft.

♪ So, this is the hatch where the crew would access the Lancaster.

Down there, the old rear gunner, tail-end Charlie, armed with machine guns.

Not much use against enemy fighters, but for this particular mission, for the Dambusters raid, he'd be aiming at targets on the ground with like searchlights and anti-aircraft batteries, having to retrain himself to do that.

No upper turret on the Lancasters that went in the Dambusters, because that was taken out to reduce weight.

And we come forward here.

It's very, very difficult to move around in these aircraft, particularly if you're a bit tall, like me, but I've come through now to where the rest of the crew were.

Here we go.

Whoa!

Wireless operator.

And then we come forward to be with the navigator right here, looking at his maps.

And we come up to the cockpit itself.

On these Lancasters in Bomber Command by this period, there was only one pilot.

He sat here.

There just weren't enough pilots to go round, so high was the loss rate, the attrition rate of pilots.

He had armor plating on this chair to try and keep him safe, but if he goes, the whole crew are in big trouble.

There was an engineer sat beside the pilot.

And then we if we go forward into the nose of the aircraft, that's where the bomb-aimer lay there, and above him, there would be a forward-facing gunner as well so that he could engage enemy targets on the ground.

(soft music) ♪ On Wednesday, the 31st of March, the crews of 617 Squadron finally begin their intensive training for the mission.

As they climb into their mighty Lancasters, the crew know this mission is going to push the aircraft to its limits.

(dramatic music) ♪ Because the squadron now has to learn to fly at extreme low level to avoid being picked up by German radar and become easy prey for Nazi fighters.

♪ (engine buzzing) All the crews of 617 Squadron have flown Lancasters before, but now, they're going to have to completely relearn how to fly that aircraft, 'cause it was meant to operate at 15,000 to 20,000 feet.

They were now going to fly it at treetop height.

(engine buzzing) (Onyeka) This operation, to be successful, required a number of skills on the part of the pilots.

First of all, they had to operate their Lancaster bombers as if they were fighter pilots in fighter planes.

But the Lancaster bomber isn't a fighter plane.

It isn't designed to fly low.

It's designed to drop bombs from a height.

So they were actually requiring of that airplane for it to do something which it wasn't created to do.

(solemn music) (Mike) Low level flying, it's exhilarating, but it is very challenging.

You're flying fast, so the slightest error in concentration or lapse in concentration means that you're very close to the ground.

So, you need to be wits about you the whole time in flying your airplane when you're at low level.

(Dan) As bomb-aimer Johnny Johnson lies on his front in the Perspex nose of the Lancaster, he has a bird's-eye view.

Johnny Johnson loves it, but he's a little bit put off when his American pilot, Joe McCarthy, flew so low over a field of flowering tulips in Spalding that he ripped up most of the farmer's crop.

♪ Another occasion, they were following the line of a canal by Sutton Bridge, and McCarthy took them underneath some high-voltage electricity cables that crossed their path.

(dramatic music) Incredibly, there are no accidents, but there are plenty of close scrapes.

♪ Some Lancasters even return from sorties with the tops of trees caught in their rear undercarriage wheel.

♪ They're flying on the edge of disaster.

♪ But that was just the beginning.

Flying low level was just part of their preparation.

They also had to learn how to navigate and drop bombs to within a few feet of a target, all whilst flying at night.

♪ It was almost impossible.

(pensive music) ♪ Despite only 44 days remaining until the mission deadline, latecomers are still joining 617 Squadron and being thrown into intensive training.

♪ The pilots are learning to fly their massive Lancaster bombers at treetop height.

♪ And the navigators must master a whole new method of finding their way to the target.

(Max) By 1943, Bomber Command's navigation techniques had become a lot better than they were in 1939, but they were still very imperfect.

They were going to have to navigate by eye in the moonlight just by following rivers and following lakes, power lines and so on.

And even though flying in at deck level meant they should escape the night fighters, it also meant they were completely vulnerable to low level German flack, machine guns, searchlights and so on.

(solemn music) (Dan) At low level, the Lancasters' primitive radio navigation system doesn't work.

♪ So now, the crews begin learning how to thread their way low across occupied Europe and Germany using just a map and a compass.

♪ (indistinct remarks) (Dan) Today, I'm in the navigator's seat of a light aircraft to see what the men had to deal with.

♪ Former RAF Pilot Graham Duff will be at the controls.

So, if you're flying up at 15,000 feet, presumably, you can see the coast, you've got the mountains over there, you sort of know where you are, which you don't get when you're at low level.

-It's all happening pretty fast.

-Yeah, so, the perspective is totally different.

And because things look different, you now need to navigate by different features.

(soft music) (Dan) Our destination: the Derwent Reservoir in the Derbyshire Peak District.

A training area used by 617 Squadron.

♪ Before every flight, the navigators had to plot a series of landmarks on a map, which they used as turning points on route to the target.

Miss a turning point, and they could become completely lost.

(tense music) ♪ (engine buzzing) ♪ Today, I might not be flying in a Lancaster bomber.

But I'm going to try using the same low level navigation methods as the Dambusters.

But if I get lost, Graham will take over.

♪ This is the exact countryside they flew over at low level.

Many of these houses, barns, and farms haven't changed much since.

There's probably a few people down there that remember the Lancasters going over.

(Graham) We're going to descend down low level.

-Watch out for the trees.

-Yeah.

Trees I reckon about 30 to 50 feet.

Now, we've got power lines to worry about.

(Dan) Yeah.

The lowest we're allowed to fly on route is 100 feet.

And I'm struggling to locate my first turning point.

(soft music) I've already lost my bearings.

♪ We're about two miles east of where--on the line, so my timing is going to be a bit off.

(Graham) Yeah, timing's off.

♪ (Dan) If 617 Squadron made a similar mistake during the mission, it would likely be fatal.

♪ Life comes at you fast when you're navigating at low level.

The landscape opens up.

You got to keep readjusting, revaluating where you are, not to mention avoiding electricity pylons.

(solemn music) As we enter the valleys near our target, accurate navigation is vital.

♪ The Dambusters knew if they strayed off course, they could crash into the side of a mountain.

♪ You've got to have quite an iron stomach, because it's quite bouncy down at this level as well.

Because the wind's following the curves in the ground, which is causing all this turbulence.

It just adds an extra level of complexity and unpleasantness to the navigator's job.

(Graham) Imagine trying to doing this in a Lancaster.

It's hard enough in this airplane.

(Dan) Yeah.

♪ After a few wrong turns and a very bumpy final approach...

There, this is the famous valley now.

We've found it.

We have found it.

♪ We've located our target.

♪ This practice run to Derwent was only 40 miles, but to reach the Ruhr Valley Dams, 617 Squadron must navigate low level across 400 miles of heavily-defended enemy territory.

(soft music) ♪ The squadron now have just six weeks until the mission.

♪ They've completed 26 daylight navigation practice runs and also started practicing precision low level bombing.

♪ Because for the bouncing bombs to work, Barnes Wallis believes they have to be dropped exactly 150 feet above the reservoir at a precise distance from the dam wall.

Although the crews still have no idea what their weapon or target are, they do know they'll have to attack over water.

♪ So to practice, the crews drop non-bouncing dummy bombs on Derwent Reservoir.

♪ This is where the aircraft are approaching, low level.

Flying at around 240 miles an hour at just 150 feet off the ground or the surface of this water.

In the middle there, bobbing, there's a raft, and they have to try and drop the bombs as close to that raft as possible.

And there's a gentleman here on the side of the reservoir with a pair of binoculars carefully noting down just how close the bombs get to that raft.

(mellow music) But the results are disappointing.

The Lancaster's bomb-aiming equipment, designed for use at 15,000 feet, is useless at low levels.

♪ (engine buzzing) And training is about to get even more difficult and dangerous.

♪ So far, it's taken place during the day, but Gibson knows that on the mission, they have to fly low level across Europe and drop their bombs at 150 feet, all at night.

♪ But he isn't willing to let his aircrews practice this without first attempting it himself.

♪ So in late afternoon, he sets off from Scampton with two of his most trusted pilots alongside him in the cockpit.

♪ Gibson and the other pilots head here to the Derwent Reservoir.

It's a hazy day, like today, but they manage pretty well flying at 150 feet.

They do it again and again, but as nightfall approaches, things get a whole lot more difficult.

(solemn music) As the suns sets, Guy Gibson descends one more time.

But the blue waters of the reservoir have turned inky black.

It's impossible to judge his height.

On his last run, Gibson comes in too low.

He's just a few feet from the surface of the reservoir just there.

If he hits the water, it's certain death for him and his crew.

He realizes his mistake just in time.

He pulls the aircraft into a climb.

His bomb-aimer shouts, "Bloody hell, this is dangerous!"

♪ 617 Squadron has just come within a few tenths of a second of losing its squadron commander and two of its most senior pilots.

Those losses surely would have led to the instant cancellation of the dams' raid.

♪ There are still six weeks remaining, and the mission is in trouble.

617 are struggling to fly their Lancasters at extremely low level at night.

And they still can't drop their bombs with the necessary precision.

♪ It seems an impossible mountain to climb.

♪ Next time, we count down the tense final days to the raid itself.

When the full-scale bomb fails catastrophically... Barnes Wallis groans, "Oh, my God."

...the mission is thrown into jeopardy.

The increasingly dangerous training almost ends in disaster.

The bomb crashes onto this floor of the hangar.

People run for their lives.

And the target itself is finally revealed to the crews.

♪ (dramatic music) ♪