Area 51

Area 51 was/is an experimental airbase. The reason they guard it so closely isn't because of aliens but rather that it's the place they test very sensitive aircraft and similar things that have the potential to change warfare to a fundamental level. The SR-71 was developed there and tested many times. They also tried to develop a flying saucer (there is a video that shows someone driving a floating circular disk) that Hitler had also tried to develop. They didn't go through with it because the scientists decided that it the time would be wasted to try and make an actual high altitude one.