The United Kingdom, along with the United States and the Soviet Union, signed the Partial Test Ban Treaty, which restricted it to underground nuclear tests by outlawing testing in the atmosphere, underwater, or in outer space, on 5 August 1963. The UK signed the Comprehensive Test Ban Treaty, ending all nuclear testing, on 24 September 1996, and ratified it on 6 April 1998, having passed the necessary legislation on 18 March 1998 as the Nuclear Explosions (Prohibition and Inspections) Act 1998. Subcritical nuclear tests continued to occur, most notably the Etna test in February 2002, and the Krakatau test in February 2006.