Germany wanted to invade England with tanks, made them air tight with massive hoses so they could go right under the English strait.

Fun thing is the underwater tank part worked, problem was the English strait is... Not a flat or hard surface. The tanks had to go through terrain they were in no way able to navigate, in the near pitch black of the deep water and if they stopped they'd sink into the silt of the sea floor.

Then the crew would have to swim for it. I have no idea if the strait is deep enough to give you the bends but fuck that must have been miserable.