Shot Umbrella was a DOD-sponsored underwater detonation conducted on 9 June 1958 as part of Operation Hardtack I in the Enewetak lagoon. A Mk-7 bomb was suspended at 46 metres below the surface in a heavy pressure vessel and detonated on the lagoon floor north-northeast of Mut Island, producing 8 kilotons. The test was designed to study the effects of a medium-depth underwater nuclear explosion — generating an underwater crater approximately 900 metres across and 6 metres deep. Shot Umbrella was one of only a small number of underwater nuclear tests ever conducted by the United States, and the data it produced informed naval planning for the use and defence against nuclear depth charges.