British frogman Lionel Crabb prepares for dive mission in 1950

Commander Lionel Crabb, a veteran of undersea missions during World War Two, prepared for a mysterious dive with enough oxygen to last several hours. His mission was not publicly known, and his appearance caused diplomatic tensions. Despite England's statement of no permission for the dive, Crabb was never seen alive again, leaving his fate a mystery.

How long was Commander Crabb expected to remain underwater?

What is known about the fate of Commander Lionel Crabb?