Construction of a large underground site in Yeongjeo-ri began in the mid-1990s, and it was thought to be a missile base only. However, it is in a mountainous area and is well hidden, making it a potential site for other clandestine activities.[1] In October 2002, North Korea admitted that it possessed a program to enrich uranium, and this site was named as one of three sites suspected for this program, along with Hagap and the Academy of Sciences in Pyongyang.[2]

Sources:
[1] Yang Nak-gyu, “북핵실험 5대 의혹 장소는 [5 Suspected North Korean Nuclear Test Sites],” Asian Economy, 14 April 2011, www.asiae.co.kr/news.
[2] Choi Hyun-soo and Ahn Ue-geun, “북 핵 은닉 장소, 평북 구서.영변 일대… 군당국, 8 -13곳 주시 [8-13 Hidden North Korean Locations Suspected of Being Nuclear Facilities],” Kukmin Ilbo, 19 September 2009, via: www.kinds.or.kr; “북핵 개발 ‘의심’ 시설 3곳 [3 Suspected North Korean Nuclear Development Sites],” Yonhap News Agency, 19 October 2002, via: www.kinds.or.kr.