DIVE SITE: There’s a mysterious bomber at the bottom of Lake Mead. “The Overton Arm B-29 — named for the part of Lake Mead where it crashed in 1948 — tested a secret ballistic-missile guidance system. The nature of the test required a risky and dangerous flight. But a historic drought has lowered Lake Mead’s levels, and may soon give recreational divers access to the sunken bomber.”