<P> The first nuclear device was detonated as a test by the United States at the Trinity site on July 16, 1945, with a yield approximately equivalent to 20 kilotons of TNT . The first thermonuclear weapon technology test of engineer device, codenamed "Ivy Mike", was tested at the Enewetak Atoll in the Marshall Islands on November 1, 1952 (local date), also by the United States . The largest nuclear weapon ever tested was the "Tsar Bomba" of the Soviet Union at Novaya Zemlya on October 30, 1961, with the largest yield ever seen, an estimated 50--58 megatons . </P> <P> In 1963, three (UK, US, Soviet Union) of the four nuclear states and many non-nuclear states signed the Limited Test Ban Treaty, pledging to refrain from testing nuclear weapons in the atmosphere, underwater, or in outer space . The treaty permitted underground nuclear testing . France continued atmospheric testing until 1974, and China continued until 1980 . Neither has signed the treaty . </P> <P> Underground tests in the United States continued until 1992 (its last nuclear test), the Soviet Union until 1990, the United Kingdom until 1991, and both China and France until 1996 . In signing the Comprehensive Nuclear - Test - Ban Treaty in 1996, these states have pledged to discontinue all nuclear testing; the treaty has not yet entered into force because of failure to be ratified by eight countries . Non-signatories India and Pakistan last tested nuclear weapons in 1998 . North Korea conducted nuclear tests in 2006, 2009, 2013, 2016, and 2017 . The most recent confirmed nuclear test occurred in September 2017 in North Korea . </P> <P> Nuclear weapons tests have historically been divided into four categories reflecting the medium or location of the test . </P>

When was the last time a nuclear bomb was detonated