Bikini Lagoon, 20 July 1946, Test Baker minus 5. As with the Queen Day rehearsal for Able, all went well on 19 July at the William Day rehearsal for Baker, though the “dummy” bomb – a flash bomb containing no atomic material – exploded prematurely startling everyone.
Baker would be the first underwater explosion of an atomic bomb. The Baker bomb – nicknamed “Bikini Helen” or “Helen of Bikini” – would be lowered in a caisson and submerged 90 feet below the target vessel LSM-60 (Landing Ship, Medium). LSM-60 was refitted extensively with special rigging facilities, a laboratory, and radio and electronic equipment. Wire cable, electrical wire, and a coaxial umbilical, which would transmit the ultrahigh frequency signal that would detonate the bomb, would link the bomb to the ship. The arming would be done remotely from a support ship by a sequence of radio impulses … but not until it was ready to be fired and all men were safely outside the danger zone.
(Excerpted from Operation Crossroads – Lest We Forget! by William L. McGee.)