Bikini Lagoon, 20 July 1946 – As with the Queen Day rehearsal for Test Able, all went well on 19 July at the William Day rehearsal for Test Baker, even 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.)