This seven-story giant boulder has attracted UFO conferences, Hopi spiritualists, and the engineers of a “rejuvenation machine.”

Geologically speaking, Giant Rock—located in California’s Mojave Desert—is roughly seven stories high and covers almost 6,000 square feet. Some say it is the largest freestanding boulder in the world.

While the rock has been a Native American spiritual site for thousands of years, the modern backstory of the boulder begins in the 1930s, when a German immigrant and miner named Frank Critzer met a pilot named George Van Tassel. The pair became fast friends and Van Tassel loaned Critzer 30 dollars to buy mining equipment. Critzer then dug out a 400-square-foot home for himself directly beneath the rock. Some locals thought he was crazy, but since he was known to point a shotgun at those who approached his underground home, no one inquired further. Critzer was also a radio enthusiast, and is said to have set up a radio antenna on top of the rock for better reception.

Unfortunately, Critzer’s German origin and radio antenna led to suspicions of his being a spy during World War II, and a police raid was made on his cavern. While the exact cause of Critzer’s death is still unknown, legend holds that when authorities attempted to extricate him by shooting tear gas canisters into his cave, one accidentally ignited a small store of explosives (for mining) and blew the peculiar loner to smithereens. As it turns out, Critzer was not a spy after all, but just what he seemed: an eccentric who wanted to be left alone to live, quite literally, under a rock.

Upon hearing of his friend’s death, Van Tassel—a high school dropout who had become a pilot—went to the boulder and reopened an old airfield at the Giant Rock in the 1950s, naming it Giant Rock Airport. Van Tassel’s war friend Howard Hughes, for whom Van Tassel was a test pilot, is said to have flown there just for a slice of pie baked by Van Tassel’s wife.

In addition to being an aviator, Van Tassel was also a firm believer in alien life. In 1952, Tassel began holding meditation sessions in Critzer’s old home under Giant Rock. Here, Van Tassel believed he was receiving vital information from alien sources directing the construction of a fantastic machine. The body, Van Tassel learned from his alien sources, was an electrical device, and aging was caused by a loss of power. Van Tassel claimed to have even been transported to an alien spaceship, where he met a wise group of aliens known as the “Council of Seven Lights.” Van Tassel said this extraterrestrial meeting, along with ideas from scientists such as Nikola Tesla, inspired the construction of a building/device which was to be a “rejuvenation machine.” It was dubbed “The Integratron.”

Van Tassel held popular UFO conventions known as the “Giant Rock Spacecraft Conventions” on his property for over 20 years to help raise money for the Integratron’s construction. The domed structure, built without nails over a period of 34 years, was said to be capable of collecting up to 50,000 volts of static electricity from the air in order to charge the human body. Unfortunately, Van Tassel suffered a heart attack before its “final” completion, giving rise to a host of conspiracy theories. There were once plans to turn the Integratron into a disco, but instead it has been reincarnated as a sound bath meditation retreat.

Long before Van Tassel or Frank Critzer were around, Giant Rock was a spiritual site for thousands of years, used by Native American tribes in ceremonies and prophecy. Hopi shamans have suspected since the 1920s that the future of the 21st century would be foretold at Giant Rock, based on how the rock cracked. In February 2000, a giant chunk of the rock did indeed break off. Spiritual leader Shri Naath Devi interpreted the break in a positive light, saying, “the Mother had opened her arms to us, cracking open her heart for the whole world to see.” It is speculated the break was the result of fires burned under the giant rock in what was once Frank Critzer’s underground home.