On July 21, 1948 a Boeing B-29 Superfortress bomber ditched into Lake Mead. Apparently the pilots, on a test flight, misjudged their height over the water and hit the surface at 250 mph. The crew were able to escape into life rafts before the plane sank and were rescued, but the plane was never recovered. This was one of WWII's largest planes, and it's down there nearly intact.
Lots more about it here:
And here's a link to all of the National Park's Lake Mead videos