How nuclear testing moved from open skies to sealed shafts underground
The first atomic test lit the New Mexico desert on July 16, 1945, with the force of about 20 kilotons of TNT. Over the following decades more than 2,000 tests followed, first on towers, barges and islands, then high above the atmosphere, and finally deep underground, as worries about global fallout forced the explosions out of sight.
Escalation came quickly. The United States set off Ivy Mike, the first engineered thermonuclear device, on November 1, 1952, at Enewetak, a Pacific atoll in the Marshall Islands. The Soviet Union's Tsar Bomba, detonated over Novaya Zemlya on October 30, 1961, remains the largest ever tested, at an estimated 50 to 58 megatons. Blasts near the ground sucked soil and debris into their mushroom clouds, turning it radioactive and scattering fallout far and wide.
Rising alarm fed an anti-nuclear movement, and in 1963 Britain, the United States and the Soviet Union signed the Limited Test Ban Treaty, giving up tests in the air, underwater and in space. Going underground remained allowed, so it became the norm for the superpowers. Properly contained shots release almost no fallout, though some vent to the surface, and every one shakes the ground and usually leaves a sunken crater. A 1976 treaty capped underground yields at 150 kilotons. France kept testing in the atmosphere until 1974 and China until 1980.
The other categories had odd side effects. Explosions lofted by rockets into the ionosphere could produce an electromagnetic pulse, and charged particles from them could travel along magnetic field lines to the opposite hemisphere and paint auroras. Underwater shots, such as those of Operation Crossroads, tested what the weapons did to warships. Tests also had many aims beyond checking a design: measuring effects on buildings and equipment, confirming a weapon could not detonate by accident, improving detection, and, under the American Operation Plowshare, exploring peaceful uses.
Underground testing ended with the Soviet Union in 1990, Britain in 1991, the United States in 1992, and China and France in 1996, the year the Comprehensive Nuclear-Test-Ban Treaty was signed. It still has not entered into force, and the last confirmed test took place in North Korea in September 2017.
Source: Nuclear weapons testing
