Historic image of a nuclear weapon being detonated. "Operation Hardtack I" was a series of 35 nuclear tests conducted by the United States in 1958 at the Pacific Proving Grounds. In 1958 the arms race proceeded apace, with the enormous weapons production infrastructure and both weapons labs operating at full speed. Partly this burst of testing activity was due to building pressure for an imminent test moratorium, leading the weapons labs to rush as many device types to the test range as possible. A total of 35.6 megatons were shot during this series. The lab tests centered on ICBM and SLBM missile warheads and high yield strategic bombs. The DOD conducted high altitude multi-megaton tests to study their usefulness for ABM (anti-ballistic missile) warheads, and discovered the high-altitude EMP (electromagnetic pulse) effect in the process. Effects tests of underwater explosions were also conducted.