B-57 bomber with the nuclear bomb in the backdrop, from Operation Hardtack I, with an 8.9 megaton nuclear explosion.  Operation Hardtack I was a series of 35 nuclear tests conducted by the United States from April 28 to August 18 in 1958 at the Pacific Proving Grounds.   At the time of testing, the Operation Hardtack I test series included more nuclear detonations than all prior nuclear explosions in the Pacific Ocean put together. These tests followed the Project 58/58A series, which occurred from 1957 December 6 to 1958, March 14, and preceded the Operation Argus series, which took place in 1958 from August 27 to September 6. Check it out: