August 5th, 1963 Three of the four nuclear powers sign a limited treaty that bans most, but not all, nuclear weapons testing.
The 1963 treaty only banned "Atmospheric" tests (in the air, space, and underwater) not "most". Testing just moved underground and continued at about the same pace, maybe even faster for a while.
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The US was the first to start testing nuclear weapons
1943
underground
underground
The US performed the first nuclear weapon test on July 16, 1945.