Manson may have learned of the underground world from the story of Tom Wilson, a Cahroc Indian who was a Death Valley guide in the 1920s. Wilson said that when he was a boy, his grandfather told him that he had found a tunnel that extended for miles beneath the valley. Walking its length, the man ended up in an underground chamber where a race of fair-skinned people dwelt.
Welcomed by these subterranean humanoids, Wilson's grandfather lived with them for a while. The people spoke a strange foreign language, wore clothes made of a leather-like substance, and illuminated their home with a pale greenish-yellow light of unknown origin.
The Indian eventually resurfaced and returned to his people, who were understandably skeptical about his adventure. But Tom Wilson believed that the old man hadn't lied, and he spent the rest of his life searching for the entry to this underground world, convinced until his death in 1968 that it actually existed somewhere underneath Death Valley.
At one point Wilson teamed up with a prospector named White, who claimed that he too had found strange underground dwellings in Death Valley. White had been exploring an abandoned mine in Wingate Pass when he fell into a hidden tunnel that led to a series of rooms.
The rooms were filled with leather-clad human mummies. Gold bars and other fabulous treasures were stacked in piles around them. There was a passageway leading beyond the rooms as well, lit by an eerie greenish-yellow light. But White dared not explore any further, fearful of what might lie beyond.
White visited the rooms three more times, once with his wife and once with another prospector. But he was unable to locate the cavern later when accompanied by Wilson and a group of archeologists, although they did find a curious dead-end tunnel into the solid rock. The area around Wingate Pass was eventually absorbed into the China Lake Naval Weapons Center, and is now closed to the public.
Taken from weirdus.com