The Hardtack Umbrella event was a Los Alamos and Dept of Defense nuclear weapons explosion shallow depth underwater shot (150 ft.), June 8, 1958, at Enewetak. The test was weapons effects related and the yield was 8 kilotons.

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The Hardtack Umbrella event was a Los Alamos and Dept of Defense nuclear weapons explosion shallow depth underwater shot (150 ft.), June 8, 1958, at Enewetak. The test was weapons effects related and the yield was 8 kilotons.
'Hardtack Umbrella' underwater nuclear test
Operation HARDTACK I consisted of 35 nuclear tests conducted at the Pacific Proving Ground between April 28 and August 18, 1958. These tests included balloon, surface, barge, underwater, and rocket-borne high-altitude tests.
Umbrella was a DOD sponsored weapons effects test for a medium depth underwater explosion. A Mk-7 bomb was used for the test (30 inches in diameter, 54 inches long, device weight 825 lb.) in a heavy pressure vessel (total weight 7000 lb.). Very similar to the Wahoo device. The device was detonated on the lagoon bottom NNE of Mut (Henry) Island. An underwater crater 3000 feet across and 20 feet deep was produced.
The purpose of these tests was to improve the understanding of the effects of underwater explosions on Navy ships and material.
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Eniwetok Atoll, Marshall Islands - June 8, 1958