Some of these potential alien bolt-holes are old theories, and some of these have only surfaced in recent years. Malibu is certainly one of the newer ones, only being pointed out in 2014 by a caller to Jimmy Church's "Fade To Black" radio show. Discovered on Google Earth, they realized that there was an especially perfect circular structure six miles off the coast and 2,000 feet down.
All of these anonymous tip-offs are generally considered pretty sketchy, but this one actually turned out to be completely true, and completely incredible. Sonar images show that there is indeed a coliseum-like structure, three miles in diameter, hidden in the Californian depths. Scientists are yet to take the plunge and investigate fully - maybe because they're scared of what they'll find...