Hi Norman
That was A-Bomb Test Baker, the 5th atom bomb detonated after Trinity, Hiroshima, Nagasaki, and test Able. This was the first underwater detonation. Trinity was an airburst on a tower 90 feet up, the two Japanese bombs and the Able test shot were also airbursts at about 1500 feet if I recall. The Able shot tested an airbursts potential against a potential enemy ship fleet, which was made up of captured and decommissioned warships, including the Prinz Eugen and the Nagato, as well as several veteran American ships that deserved a better send off. That test damaged and sunk several ships but surprisingly most survived. Baker was to test the effects of an undersea burst and the effect of the underwater pressure shock wave on surviving surface ships, the bomb was dangled on an LSTs anchor chain 90’ in a 180’ deep bay, the blast completely disintegrated the LST, no trace of it was ever found, and if you watch the film a large black vertical mass momentarily appear in the column of water, that is the battleship USS Arkansas, sister to the USS Texas, the blast upended her vertically and then slammed millions of tons of water on top of her, she then sank almost instantly, other ships suffered massive damage from the compression wave in the water and also soon sunk.
The ships were all abandoned, the only “personnel” being test animals, pigs goats rats, some of which were instantly killed by the blast but most of which died later from radiation exposure. I have vivid mental images of test footage of little piggy fireballs.
The nearest naval personnel were 10 miles away on the other side of the atoll. That was considered a safe distance even into the early H-bomb era until the castle Bravo H-bomb test about 10 years later, the forward crews got figuratively and very nearly literally fried when the experimental nuke produced a fireball 3X larger than planned. The expected yield was estimated to be 4-5 megatons, the actual was 15 megatons! They survived safely but it was a terrifying experience for them.
Castle Bravo was also the source of the “Lucky Dragon” incident when a Japanese fishing boat was caught in the fallout.
History is fun!