Saw an interesting show called Modern Marvels on the history channel, where they were counting down some of the weirdest gadgets... This was I think #4.
In theory, the bomb would skip over the water, until it came to the dam. Then it would roll up it, roll back down underwater, and explode.
The reason this bomb was thought of is because the RAF needed to shut down dams that were providing power to the German military. Dropping a bomb on a dam was just too hard. It had to hit directly on it, or it would fall into the water and do nothing. They also could use torpedoes, but Germany had set up torpedoes nets to stop this.
So this was their idea.
http://www.youtube.com/watch?v=5JqBiMgC2Wg
