The tunnel, which extends 230 feet (70 meters) into the side of a canyon wall and is some 300 feet (91 meters) underground, was not part of the original US Manhattan Project that produced the word’s first atomic bombs and which used the Los Alamos site as its primary research laboratory.

Built between 1948 and 1949, the Tunnel Vault was “used to store the United States’ [nuclear] stockpile for about nine months, from about April of 1949 until May of 1950,” Glen McDuff, a former weapons scientist in the facility who later operated a classified museum there, says in a video released by Los Alamos.

After that it was also used to conduct weapons research and development, to store weapons components, and to assemble nuclear material assembly for both aboveground and underground tests in the Pacific and Nevada, according to Los Alamos.

“This facility had a long history during the Cold War. So this was definitely a top secret, very secure location,” Ellen McGehee, the laboratory’s historic facilities manager, says in the video.

The facility was outfitted with a forbidding security perimeter that included a guard tower with gun ports and bulletproof glass. At the end of the tunnel there is a bank vault door that leads to storage rooms guarded by a series of other vault doors.

“Any place where you need isolation, shielding, it was a really useful, handy thing to have around the lab over the years,” McDuff says in the video.

The tunnel was “fully active” until a devastating May 2000 forest fire in the area that damaged structures at the Los Alamos facility, McDuff adds.