Semipalatinsk Test Site, Soviet nuclear testing site, c.6,950 sq mi (18,000 sq km), NE Kazakhstan, near the city of Kurchatov and some 90 mi (150 km) W of Semey (formerly Semipalatinsk). Between 1949 and 1991 this area of dry steppe was the USSR's primary nuclear testing site. More than 100 tests were conducted aboveground, with significant radioactive fallout on nearby populations, until atmospheric tests were banned in 1963; more than 300 underground tests were then conducted in tunnels in the Degelen Mts. The closing of the site, collapse of the USSR, and withdrawal of Russian forces from it left a few hundred Kazakh soldiers guarding it. Large areas contaminated with plutonium and enriched uranium were eventually covered over by steel-reinforced concrete, and by 2000 most tunnels had been sealed. Many tunnels on the unsecured site were reopened by 2004 by scavengers, however, prompting concerns that radioactive material could fall into the hands of terrorists. The United States, Russia, and Kazakhstan have since cooperated to secure and clean the site.