Skipping a 9,000-pound explosive cylinder across a guarded reservoir at 60 feet in the dark sounds like a rejected movie script, but it was British engineer Barnes Wallis's answer to an impossible problem. This episode explains why the Ruhr dams and ships like the Tirpitz were protected against both torpedoes and gravity bombs, how A. R. Collins's 1942 test at the Nant-y-Gro dam in Wales proved that a small charge detonated underwater against masonry could shatter it through the bubble pulse effect, and why Wallis's April 1942 paper proposed a bomb that would bounce over torpedo nets and sink against the dam wall.

We dig into the physics of the 500 rpm backspin that let the weapon skip like a golf ball, crawl down the concrete face, and bleed off speed relative to the aircraft, then follow the shattered spherical prototypes at Chesil Beach  ...