Many people, such as the deceased Philip Schneider, believe that the government has constructed numerous underground tunnels that connect to underground cities. These underground tunnels and bases are funded through the secret black budget of the government. These bases have been in existence since at least World War II, and some believe much, much earlier than that.

These underground networks are used for various secret government operations such as extraterrestrial and terrestrial experimentation. Many of these tunnels and facilities are also said to be under key locations such as Area 51, Dulce, New Mexico and the Denver Airport.