In World War II, the Germans would frequently make use of dams they controlled as a weapon. River crossings are notoriously difficult operations, and controlling the level of a river via control of a dam mean they could make it easier for their forces to cross a river, and well nigh impossible for the Allies to cross. The dams in the Ruhr were also key contributors to the German electrical grid.
The answer to that was, well, destroy the dams. Or at least, destroy the gates to the spillways.
Because the dams were frequently well behind the lines, dozens or even hundreds of miles away, airpower was the only way to possibly reach them.
But dams make lousy targets for conventional bombing. First, they’re very hard to hit. Second, they’re tough targets, what with all that concrete.
In Great Britain, one of the foremost aviation design experts, Sir Barnes Wallis, came up with an interesting solution- the skip bomb. Wallis designed a bomb that looked something like a giant steel drum. Much like skipping a stone across a pond, the skip bomb was designed to skip across the surface of a reservoir, and when it reached the dam, it would simply sink to a pre-determined depth, and explode, while still in contact with the dam itself.
As it turned out, the RAF found out that you also had to have the bomb rotating at a pretty fair speed. If not, the bomb would skip quite energetically, hitting the bomber and knocking it out of the sky. So a bicycle chain arrangement was used to rotate the drum just before drop.
617 Squadron famously became known as the Dambusters after their gallant raid on the Ruhr in Operation Chastise.
In 2013 Cambridge engineer Dr. Hugh Hunt decided to recreate the unique bomb (on a somewhat smaller scale) and blow up a purpose built damn in Canada to demonstrate just how the system worked.