Nuclear weapons testing never really went away; it just has been hidden from public view. Once a major impetus to the "Ban the Bomb" movement as nuclear test explosions rained fallout across the globe from 1945 to 1962, U.S. nuclear testing went underground as a result of the Limited Test Ban Treaty, signed in 1963, which banned full scale nuclear explosions in the open air. Then, as now, the nuclear weapons establishment insisted on a wide range of "safeguards" to assure the continued ability to develop nuclear weapons after full-scale testing was restricted, including extensive research facilities at the nuclear weapons laboratories and a continuing program of underground nuclear tests, hundreds of which were conducted in subsequent years. The Limited Test Ban Treaty did little to restrict the ability of the United States and the U.S.S.R. to refine their nuclear arsenals, with smaller, lighter warhead designs tested underground leading to the hair-trigger arsenals of the present, replete with multiple warhead, highly accurate missiles deployed on an assortment of sophisticated delivery systems.

         Today, the cycle of nuclear weapons design continues, despite the fact that the United States last exploded a nuclear weapon underground in 1992. The Nevada Test Site remains both in readiness for resumption of underground testing and in use for a wide range of weapons experiments, including "subcritical" tests in which high explosives and plutonium are exploded underground without a self- sustaining nuclear reaction. Similar tests are conducted in steel tanks above ground at the Los Alamos National Laboratory, using an isotope of plutonium with a higher critical mass than that used in weapons. This procedure may allow weapons designers to use test devices which more closely resemble nuclear weapons primaries, the first stage of thermonuclear warheads. Although these are tests only of materials and components rather than full nuclear warheads, the Stockpile Stewardship program of which they are a part is intended to provide increasingly advanced capabilities to integrate data from a variety of testing techniques into simulations of nuclear weapons performance. (See The Stockpile Stewardship Program).

         When conducted underground at the same site used for full-scale nuclear weapons tests, subcritical experiments make verification of a test ban more difficult, and manifest to the world both the existence of a vigorous nuclear weapons research program and the intention to retain the capability for full-scale underground tests. While no verification regime can provide absolute certainty, closing all nuclear test sites and terminating "subcritical" tests which can resemble nuclear explosive tests when monitored from a distance would help simplify verification, while increasing international confidence that the nuclear weapons states were scaling back their weapons development efforts.