During World War II, German hydroelectric dams were lucrative targets for the Allies. Not only would busting one cut off a major source of power for Germany, but it would also cause destruction through flooding. Plus, it would bring the fight to Germany, rather than defending the Allies’ territory.

Finding a method for destroying the dams, however, proved difficult. Anything but a direct hit with a normal bomb would not do enough damage to break the dam, and bombs were not accurate enough at the time. A bomb powerful enough to destroy a dam via a direct hit would be too heavy for any conventional airplane to carry. On top of that, the dams were protected underwater by torpedo nets, so the only possible way to hit the dam would be against its wall, near the surface of the water. There were no bombs that could get around these problems. The British military found this dilemma unsolvable.

Then along came Barnes Wallis, who invented a bomb that capitalized on the same physics as skipping a stone over water.

Barnes Wallis was working with a team at Vickers in 1941 when he first started to work on a dam busting bomb. In the beginning he ran into the same problems as mentioned above. Eventually he got the idea of skipping a bomb over water from when the Royal Navy used to skip cannonballs across the water to increase their range. A bouncing bomb could glide over the torpedo nets and get close to the bridge – and then once it hits the wall, it could have a delayed explosion so as to allow it to slide a bit underwater to hit the dam at a more damaging location.

Wallis’ original “Highball” design was spherical, but Wallis could not get it to work in time. Instead, the bomb used on the mission was an “Upkeep,” which was cylindrical. During its deployment the bomb would be spun backwards at a high rate (500 rpm), then skipped across the water.

Unfortunately, to achieve the desired skipping stone effect and destroy a dam, the bomb would have to be dropped at a very low altitude (exactly 60 ft), at a precise speed (about 235 mph), and a specific distance from the target (400-450 ft). As such, only the best pilots flew for this strike, and trained to use its special equipment – not only was there such a unique bomb, but they had to be aimed using specialized aiming equipment to calculate the altitude and distance.

The mission, code named Operation Chastise, was executed on May 17, 1943. There were 19 aircraft that flew in three formations to hit six dams; the first two formations attempted to break the key dams, with the third on reserve to take a second shot at any dams that might not have been destroyed the first time around. The results were a success, despite heavy allied casualties (of the 19 planes, only 11 returned). Two of the key targets, the Moehne and Eder dams, were busted. Not only did this cause a loss of power and flooding, but pictures of the busted dams served to heighten the morale of the allied world.

Unfortunately, the destruction of the dams was not as devastating to the Germans as was hoped. Within two months water output was restored and electricity levels returned to full power. Many of the lives lost due to flooding were actually prisoners of war rather than enemies.

After Operation Chastise, the bombing squadron was kept together as a special bombing unit that specialized on precision targets with unique bombs. They would later use other unique Wallis bombs, such as the Tallboy (which was designed to break large concrete structures) and the Grand Slam (a ten ton earthquake bomb which holds the record of the heaviest bomb ever used in conflict). The squadron is still active to this day.