Hardtack-Umbrella test June 8, 1958. Fired 150 feet underwater on lagoon floor, 8 kiloton explosive force. Crater 3000 feet across, 20 feet deep.
See also what the nuke tests looked like when fired at night, from 800 miles away in Hawaii. Test name is Starfish Prime. It was an actual ICBM test firing, Titan missile.
http://nuclearweaponarchive.org/Usa/Tests/Dominic.html