George “Johnny” Johnson flew forty missions under McCarthy as a bomb aimer. “Gibson thought a lot of [McCarthy] apparently,” says Johnson, “and asked him would he join this new squadron for one trip. And Joe asked us and we said, ‘yes, fine,’ we’d go with him and so we did.”

Although Squadron 617 was formed specifically to destroy the Nazis’ most important hydroelectric dams with Barnes Wallis’ bouncing bomb, the airmen were not informed about nature of their mission. “We were never given any indication of what the target was until the briefing on the afternoon of the attack. So, right up till that time, we really had no idea. The only possible indication that it was a water target was the night before, the Saturday night, we had films of Barnes Wallis and he talked to us about the bouncing ball that he’d produced and films of how it operated and so on. Even then, I don’t think dams were considered by anybody in particular. The popular conjecture, of course, was shipping of some sort; probably attacking shipping of some sort.”

Instead, Squadron 617 was merely told to practice low level flying, and they spent their days barreling across the English countryside in Lancaster bombers. At the end of their exercise, they practiced bomb attacks on a pair of poles that were erected to simulate the towers on either end of the German dams. Unaware of what they were practicing for, the airmen of Squadron 617 took a fair amount of delight in cruising among the treetops. “Flying low level in a Lancaster was, I felt, absolutely exhilarating,” says Johnson. “So different from the ten, twelve thousand feet stuff that we had been doing. The old Lanc really sped across the ground and it really felt as though you were actually moving.”

Reality set in on the evening of May 16, 1943, when the men of Squadron 617 were assembled and briefed for their mission. Johnson remembers: “There was quite an exclamation. [Everyone was] a bit aghast when we found out what the targets were going to be. Surprise, surprise, as it were. We were even more surprised on our crew because we were briefed for the Sorpe dam.”

Unlike the other dams, which were constructed of concrete masonry, the Sorpe was made of earth, and required a different sort of approach. Yet, before they could worry about their bomb run, the crew of McCarthy’s plane had to make it to Germany. Their problems began before they even took off, as their plane refused to start up and the crew was forced to fly in the squadron’s reserve Lancaster. “So we whipped over there and got into that aircraft. But, in doing, in Joe’s sort of anxiety of getting out, he caught his parachute handle on an extension in the aircraft. And so his parachute was floating all the way behind him, and so we had to get another parachute for him.”

When they eventually got into the air, McCarthy’s crew was a bit more fortunate than members of Squadron 617 who encountered heavy resistance as they flew through Germany on the way to their targets. In fact, several of the eight crews lost on the mission were shot down or crashed into cables before they reached any of the dams. Yet, as Johnson recalls, the skies were clear: “No fighters. Don’t ever remember seeing any fighters, let alone being attacked by them. I suppose we were just, excuse the language, bloody lucky.”

In terms of the bomb run itself, Johnson remembers that the geographical situation of the dam required them to fly lower than the other planes — they ended up dropping their bomb at a mere thirty feet! “You had to come down the hills on one side and then get up the other side quick before you hit those hills on that side. What we hadn’t been told on briefing was that, on the line of attack, there was a church steeple on the side of the hill. So Joe, in his wisdom, decided to use that as a marker and came just over the top of that and straight down. In actual fact, it took us ten runs to get it right. It was a question of getting the aircraft level, getting down to the height, and getting everything right before you needed to pull up and get to the other side. Dave [Rogers, the rear gunner] was a bit concerned that every time I said dummy run — you know, ‘oh, Christ, what next?’ He was getting the full pull of the G [force], being in the rear turret and it all going to that end. He said afterwards, ‘I was thinking of throwing that bastard out of here.’ I learned pretty quickly how to become the most unpopular member of the crew, because I was the one that was saying ‘dummy run.’ Nine times I said that, and on the tenth time we actually made the drop. And looking afterwards I couldn’t see … but Dave said the spout of water that came up was absolutely tremendous. In fact, some of it actually hit the rear turret. [Rogers] said he’d begun to think he was gonna get drowned as well as having been knocked about.”

The only real problem McCarthy’s crew encountered came after the mission, as they prepared to land: “With the modified aircraft, the mid-upper [gun] turret was taken out and the mid-upper gunner flew in the front turret. So we were toddling along, and we saw this goods train just going along very nicely thank you, and Ron Batson our front gunner said, ‘can I have a go, Joe?’ And Joe said, ‘yeah, ok, right.’ So Ron opened up with our little 303s. What we didn’t know was that it was an armored train and, of course, it gave us rather more than we were giving it. So we got out of that. We knew we’d been hit, but we had no idea where. It didn’t impede the aircraft anyway, so there was no problem as far as that was concerned. And it wasn’t until we got back that we discovered where we’d been hit. In actual fact, a piece of shrapnel had … burst the starboard tire. So that, when we landed, we were more than a little bit starboard wing down and had a bit of a jutter going on. But, again, Joe managed to control it extremely well. He really was an ace of a pilot; there’s no doubt about that. He was absolutely first class.”

As the popularity of airships faded, Wallis began working on aircraft design. His geodetic latticework was incorporated into the Wellesley bomber, which set a non-stop distance record in 1938 of 7,158 miles. The Wellesley was updated into the Wellington, the primary British bomber used during the early part of World War II. The Wellington was noted for its ability to sustain damage and remain airborne, due in no small part to Wallis’ design.

During World War II, Wallis continued his work to aid Britain’s military efforts. In his spare time, he toyed with a problem the British military had largely considered unsolvable — how to destroy the Nazis’ most important hydroelectric dams. Breaking these dams by conventional means would have required a 60,000-lb. bomb, which was far too heavy to be realistic. So Wallis devised a new kind of “earthquake” bomb — a ten-ton armament that would break the sound barrier as it fell from a height of 40,000 feet, crashing deep into the ground where it would explode, shaking the earth so violently that the dam would shatter. With this sort of bomb, a direct hit on the dam was not needed. Since the RAF had no plane capable of undertaking such a mission, Wallis also designed an enormous, six-engine bomber called the Victory that would have been larger than any other bomber in the world.

Yet the Ministry of Aircraft Production told Wallis that Britain simply didn’t have the resources to create such a plane. (Incidentally, two variations of the earthquake bomb were employed later in the war. In 1944, a six-ton version called Tallboy was used to bomb a railway tunnel, U-boat pens, naval vessels, dams, and launching sites for the German V1 and V2 rockets. In March of 1945, the Lancaster bomber was adapted to accommodate the ten-ton bomb Wallis originally envisioned. This bomb, known as Grand Slam, was used against the same targets as Tallboy, in addition to several bridges.)

At the time, however, Wallis was limited to the use of a six-ton bomb, and he began brainstorming for a way of using such a weapon to bring down a dam. Tests were conducted on a scale model of the Möhne dam, and then on the Nant-y-Gro dam in Wales which, though it was five times smaller than the Möhne, was blown up to study how the German dam might be brought down. It turned out that fewer explosives were required to destroy a dam when the explosion took place in contact with its wall. Recalling the way Admiral Lord Nelson skipped cannon shells across water, Wallis wondered if perhaps he could deliver a bomb by skipping it across the reservoir and into the face of a Nazi dam.

He began in his backyard by shooting marbles across a tub full of water with a small catapult. Eventually, he determined that the marbles needed to be launched at a seven-degree angle in order to skip across the water. On a nearby lake, he built a larger catapult and began testing projectile materials. He tried balls made of everything from balsa wood to lead, each engraved with different surface patterns. Wallis found that dimpled balls flew the furthest and skipped more often than the other balls.

Moving to a secret government facility, Wallis began firing his balls at a wall in a giant tank of water. To prevent the balls from bouncing too far away from the mock-dam after they hit its wall, Wallis added a pin to the firing mechanism on his catapult that spun the balls backwards before they were launched. As a ball bounced off the wall, this backspin killed its forward momentum, and the ball sank right alongside the wall. [The “Bomb the Dam” interactive, above, illustrates this process.] In real life, water pressure would detonate the bomb at a depth of thirty feet. Wallis built half-size bomb prototypes to test how this would work in the field. A plane was outfitted with a special rig to spin the bomb, and the experiment proved successful — the bomb skipped.

Wallis submitted his plan to Sir Arthur “Bomber” Harris, head of the British Bomber Command, who initially referred to the idea as “tripe of the wildest description.” Yet Wallis was able to convince him that the bombs would work by showing him film of his field trial, and a squadron of Lancaster bombers — Squadron 617 — was created to fly the mission. Wallis had only twelve weeks to prepare for the mission, which needed to be conducted in spring, when seasonal rains had swelled the dams. This did not allow him enough time to construct the dimpled bombs he had originally designed. It was faster to make metal, cylinder-shaped bombs with a round wooden casing.

On an April 13, 1943 test run, the new bombs sank straight into the water and did not skip. Between watching the film and conducting more test runs, Wallis discovered that if he discarded the wooden casing and dropped them from a height of 60 feet, the cylinder-shaped bombs would skip like he had planned. This meant the pilots would have to fly considerably lower than the 150-foot height at which they had begun their training, but it would have to do. Since their altimeters were not accurate at such a low height, the aircrew invented a method of determining their height with two spotlights. These lights were attached to the aircraft so that they shone down on the water at a defined angle. When the two beams of light met on the water’s surface, the aircrew knew they were flying at 60 feet.

All of the planning and last minute design-fidgeting paid off on the night of May 3. Though Wallis was delighted by the reports of the destruction his bombs wrought on the Möhne, Eder, and Sorpe dams, he was devastated to learn that 53 of the aircrew were lost on the mission; three more men survived, but were taken prisoner. He vowed never again to design a project that would put so many British lives in jeopardy. In 1968, Wallis was knighted for his service to the United Kingdom.

The commander of Squadron 617, 24-year-old Guy Gibson, had already completed 170 sorties by the time he was chosen to lead the mission. Operation Chastise was so top-secret that Gibson was at first told nothing of the task that lay before him, other than that he needed to train his pilots rigorously in the art of low level flying. Eventually, the pilots simulated the nighttime conditions under which they would fly into the Ruhr by fixing blue Perspex plastic into their cockpits and wearing amber-tinted goggles.

In his autobiography, Gibson recalled that their training was hampered by communication problems. The radio-telephone headsets they used were ineffective, resulting in chaos during a dress rehearsal for their bomb run. “Aircraft went astray, some nearly collided, others went home browned off,” he wrote. The communication snafus were solved when the entire squadron was outfitted with better equipment. Yet problems persisted. During another dress rehearsal, “six out of twelve aircraft were very seriously damaged by the great columns of water sent up when their mines splashed in.” Gibson concluded that the pilots had been flying too low.

The difficulty of their training only hinted at the danger that would confront Squadron 617 in Germany. “The gunners had seen us coming,” Gibson wrote. “They could see us coming with our spotlights on for over two miles away.” Before the planes even reached the dams, one aircraft hit cables and crashed and two more were shot down. In 1944, Gibson acknowledged how precarious flying just 60 feet over enemy territory could be. “I said quickly to Pulford, under my breath, ‘Better leave the throttles open now and stand by to pull me out of the seat if I get hit.’ As I glanced at him I thought he looked a little glum on hearing this.”

At the Möhne dam, Gibson managed to drop his bomb unscathed, although it missed the dam. Another pilot, Hopgood, followed, but his plane was hit by flak and exploded shortly after his bomb also fell off the mark. The next plane fell prey to German anti-aircraft fire, but the bombs of the fourth and fifth Lancasters scored hits, smashing apart the middle of the dam. Gibson and four other planes then flew on to the Eder, where two bomb hits created a thirty-foot-wide hole in the dam.

Unlike the others, the Sorpe dam was constructed of earth, as opposed to masonry. Joe McCarthy’s Lancaster made nine aborted runs before finally dropping his bomb. Though McCarthy and another plane scored direct hits on the crest of the Sorpe, the dam withstood the attack. The damage caused to the crest of the dam, however, required the Germans to empty its reservoir halfway and make repairs.

Although eight of the nineteen bombing crews did not return, the mission was considered a success. Four of six targets were hit; two were destroyed. One hundred and thirty million gallons of water flooded the German countryside after the destruction of the Möhne. The Eder had held back 202,000,000 tons of water, and significant flooding took place in the wake of its bombing. Miles of factories and houses were destroyed and the Germans incurred thousands of casualties. News of the operation boosted Allied morale at a time when hopes for victory were bleak.

The pilots of the RAF’s 617 Squadron crept across Germany just 60 feet above the ground, delivering an experimental weapon with astounding precision. How did these brave airmen pull off such a feat? When Barnes Wallis — who developed the bomb that broke the German dams — presented his design to Royal Air Force officials, he was laughed at. How did he convince the air force that his weapon was powerful enough to destroy a dam and clever enough to outmaneuver the underwater nets protecting these dams from traditional torpedo attacks?

In chapter 3, learn about the air raid by the Allied forces to destroy the Nazi Dams. Was the new bomb a success?

In chapter 2, learn about the development of a new type of bomb that could skip on the waters surface and destroy the Nazi Dams.

In chapter 1, learn about the dams in Germany that supplied power to the Nazi weapons manufacturing facilities and about Barnes Wallis’ proposals to effectively destroy them.

Altabba’s technical expertise has led him to work as a consultant on documentaries about historic engineering feats, such as the construction of the ancient Rainbow Bridge in China, cathedrals in Normandy, France, and now the Thailand-Burma Railway. “It is very rare that the program is something I am specifically knowledgeable about, so I end up doing a lot of my own research,” he says. “I read everything I can get my hands on historically. Usually there is very little technical stuff because engineers are typically not interested in reviewing history and commenting on previous work. They are so bogged down with their current projects that they rarely reflect back.”

In that sense, Altabba is an atypical engineer: “One of the main reasons why I’ve been asked to do these films is that I tend to get very involved in a story. I take it on. It becomes my problem. I need to figure it out. I also don’t hold anything back, so whatever I think, whatever I feel, it comes out for the cameras. The producers like that, so they try to play to it more. They really want as much of my natural, initial reaction to seeing or experiencing things so they typically keep me sheltered and limit the amount of exposure I have of the facts and the sights until I am in the middle of it, and then ‘Boom!’, on camera, I wonder, ‘My God! What are you guys putting me into?’”

During the filming of SECRETS OF THE DEAD: “Bridge on the River Kwai,” a surprise announcement by the film’s independent producers almost led Altabba to quit the project. “They did not tell me that they were going to bring in the Japanese officers [who participated in building the railway] and that I was going to be their prime interviewer,” he recalls. When I learned about it, I considered pulling out. This story, for me, was an engineering question. I was the engineering consultant and I was going to view it from an engineering perspective. I thought I should have been told, that I should have been prepared.” And yet, Altabba stuck it out. “For a day I was very upset, but then I got over it. I thought, ‘Okay, if that’s the way you want to do it, that’s how we’ll do it,’ and I went all out. I am a little on the crazy side, so I asked them the questions that were on my mind.”