A "Fat Man" Mk 3A fission bomb, dropped by B-29 "Dave's Dream", the bomb fell 298m short and 600m left of target. The USS Gilliam was sunk, and four other vessels were either sunk or severly damaged as a result of the blast. The Crossroads tests were the first nuclear explosions since World War II, and the first weapon tests since Trinity. They were "weapons effects" tests - designed specifically to study how nuclear explosions affect naval vessels, planes, and animals. A fleet of 71 surplus and captured ships was anchored in the Bikini Atoll lagoon in the Marshall Islands and used as targets.