The details of the project, published in the July issue of South Korean journal Joint Chiefs of Staff, included information that South Korean officers are being trained by U.S. Forces Korea on the facilities, Stars and Stripes reported.
Maj. Park Sung-man of the South Korean military said the United States estimates between 6,000 and 8,000 subterranean facilities have been built in North Korea for the leadership's use. Some shelters, Park wrote, are located under vacant buildings. The United States used information from North Korean defectors to map the network.
U.S. military education on the North Korean facilities began in the 1990s, and was adopted by USFK in 2007, according to Park.
Sharing of information with Seoul began in 2014, South Korean news agency Yonhap reported, because North Korean underground facilities are believed to pose serious threats to security.
The North Korean facilities include shelters and tunnels that some analysts believe are being used to produce nuclear weapons.
"[The U.S. and South Korea] need to find ways to track down and block [North Korean leaders'] escape routes when they flee Pyongyang," Park wrote.
North Korea has a long history of underground tunnel building that began in the 1960s. Pyongyang began to build decoy facilities alongside its regular passageways in order to distract outside surveillance – and projects are initiated in response to potential attacks.
In 2010, North Korea installed a large concrete barrier at the entrance of one of the underground facilities to protect against satellite-guided weapons from the South that could have been used in a counterattack after the shelling of Yeonpyeong Island, Yonhap reported.