Operation Hardtack I was a series of 35 nuclear tests conducted by the United States from April 28 to August 18, 1958, at the Pacific Proving Grounds. At the time of testing, the Operation Hardtack I test series included more nuclear detonations than the total of prior nuclear explosions in the Pacific Ocean. These tests followed the Project 58/58A series, which occurred from December 6, 1957, to March 14, 1958, and preceded the Operation Argus series, which took place in 1958 from August 27 to September 6.