Most of us have probably heard of places such as Area 51, the very secretive base in the Nevada desert, but there are a surprising amount of places on earth that are totally off limits to almost everyone. Whether that's the tiny Sentinel Island, home to one of the most mysterious tribes on the planet that will kill anyone who enters, to the highly secretive underground Russian base thought to be housing proof of alien life. Here are five places that are forbidden to the public and shrouded in mystery... And in the middle of the barren Nevada desert, there's a dusty unmarked road that leads to the front gate of Area 51. It's protected by little more than a chain link fence, a boom gate, and intimidating trespassing signs. One would think that America's much mythicized top secret military base would be under closer guard, but make no mistake. They are watching. For all the myths and legends, what's true is that Area 51 is real and still very active. There may not be aliens or a moon landing movie set inside those fences, but something is going on and only a select few are privy to what's happening further down that closely-monitored wind-swept Nevada road. "The forbidden aspect of Area 51 is what makes people want to know what's there," says aerospace historian and author Peter Merlin who's been researching Area 51 for more than three decades.