with boreholes and drifts for both subcritical and supercritical tests. After the closure of the Semipalatinsk labour camp, construction duties were performed by the 217th Separate Engineering and Mining Battalion (who later built the Baikonur Cosmodrome). Between 1949 and the cessation of atomic testing in 1989, 456 explosions were conducted at the STS, including 340 underground (borehole and tunnel) shots and 116 atmospheric (either air-drop or tower shots). The lab complex, still the administrative and scientific centre of the STS, was renamed Kurchatov City after Igor Kurchatov, leader of the initial Soviet nuclear programme. The location of Kurchatov city has been typically shown on various maps as "Konechnaya" (the name of the train station; now Degelen) or "Moldary" (the name of the village that was later incorporated into the city). The Semipalatinsk Complex was of acute interest to foreign governments during its operation, particularly during the phase when explosions were carried out above ground at the experimental field. Several U-2 overflights examined preparations and weapons effects, before being replaced with satellite reconnaissance. The US Defense Intelligence Agency is said to have been convinced that the Soviets had constructed an enormous beam weapon station at a small research station located on the testing site. This smaller research station, known to the Department of Defense as PNUTS (Possible Nuclear Underground Test Site) and the CIA as URDF-3 (Unidentified Research and Development Facility-3) was of great interest to American observers. After the fall of the Soviet Union in 1991, it was discovered that the mysterious URDF-3 was tasked with researching a nuclear thermal rocket similar to the US's NERVA. The site was officially closed by the President of the Kazakh Soviet Socialist Republic Nursultan Nazarbayev on 29 August 1991. Legacy The Soviet Union conducted its last tests in 1989. After the Soviet Union collapsed in 1991, the site was neglected. Fissile material was left behind in mountain tunnels and bore holes, virtually unguarded and vulnerable to scavengers, rogue states, or potential terrorists. The secret cleanup of Semipalatinsk was made public in the 2010s. After some of the tests, radioactive material remained on the now abandoned area, including significant amounts of plutonium. The risk that material might fall into the hands of scavengers or terrorists was considered one of the largest nuclear security threats since the collapse of the Soviet Union. The operation to address the problem involved, in part, pouring special concrete into