Lieutenant-Commander Lionel Kenneth Phillip Crabb,, known as Buster Crabb, was a Royal Navy frogman and diver who vanished during a reconnaissance mission for MI6 around a Soviet cruiser berthed at Portsmouth Dockyard in 1956.

A frogman is someone who is trained in scuba diving or swimming underwater in a tactical capacity that includes military, and in some European countries, police work. Such personnel are also known by the more formal names of combat diver, combatant diver, or combat swimmer. The word frogman first arose in the stage name the "Fearless Frogman" of Paul Boyton in the 1870s and later was claimed by John Spence, an enlisted member of the U.S. Navy and member of the OSS Maritime Unit, to have been applied to him while he was training in a green waterproof suit.