Deep underground military base
A D.U.M.B. - or Deep Underground Military Base - is a specially built underground installation hardened against external attack. Sometimes also referred to in legacy documents as a D.U.C.C. (Deep Underground Command Complex), they are designed to allow the base's core functions and personnel to carry out their responsibilities, even under extreme conditions that occur during nuclear warfare. Because of their remote and highly secure nature, these facilities are shrouded in secrecy and as a result are common plot devices in conspiracy theories.
Underground military bases are no conspiracy theory, however, dozens of such bases are known to the public with many more likely sitting undiscovered. These facilities play a crucial role in Continuity of Government or COG.
Confirmed Underground Military Installations (public domain)
PEOCx - Presidential Emergency Operations Center + Expansion, DC
The Presidential Emergency Operations Center was originally built under the Kennedy garden/East wing during the Truman remodel. Part of the bunker and extensive network of tunnels is visible in various images taken during that time.
Site R - Raven Rock, PA
Raven Rock in PA is the emergency backup operating location for the Pentagon and national military command authority.
Mount Weather - Blue Ridge Mountains, VA
Although highly classified, Mt. Weather Emergency Operations Center is believed to be the backup headquarters for the US Federal Emergency Management Agency.
Greek Island - The Greenbriar, WV
A D.U.M.B. constructed entirely in secret within a functional hotel. They disguised the construction as installation of a golf course. Employees would sometimes become suspicious of the cadre of "cable repairmen" who would show up. In actuality, they were a skeleton crew maintaining Project Greek Island, the site designed to house the entire legislative branch and keep them functional during a nuclear war. This would allow congress to maintain constitutional authority over the entire federal government even if Washington, D.C. no longer existed.
Cheyenne Mountain Complex - CO
Once served as the location of N.O.R.A.D. (North American Air Defense Command), it has since relocated to Peterson AFB.
Probable Underground Military Sites (some confirmation)
S-4 Papoose Lake
S-4 is an installation reported by Bob Lazar, Dan Burisch, Richard Doty and others. It has also been confirmed by veteran journalist George Knapp, former U.S.A.F. Captain David Fruehauf (SR-71 pilot who worked at Area 51) and independent analysis of open source data.
Dulce Base
Dulce Base is a location that was said to have been active during the 1970's and 1980's. Although suspected to have been mothballed, it was believed to have been some sort of bio-engineering laboratory. Given the totality of circumstances, investigators surmised the government was monitoring for something, although why they couldn't simply use their own cattle on their own farms is still unexplained to this date. Author Greg Valdez, son of Trooper Gabriel Valdez, believed Dulce was an off-site lab where cattle and other biologics could be taken for marking, study and whatever else they intended as part of their efforts.
Reported Locations (no supporting evidence)
Sandia Base
Sandia Base is a location alleged by UFOlogist and aviation legend John Lear. It appeared on an FAA sectional hanging on the wall of Lear's office.
Area 19 SDI Facility
Area 19 is another location brought out by Lear, except it seems to have additional confirmations that warrant taking a closer look. Satellite imagery reveals the unusual kilovolt powerline running into the ground within a double-ring of roads configuration within Area 19. Area 19 is an unusual part of the Nevada National Security Site, in that it is managed by the NNSA - that is, the National Nuclear Security Administration.