This is a sign outside Area 51, warning that deadly force may be used against anyone who enters without proper authorization. A similar sign should be placed along the border.
I haven't looked into it for a long time, but read somewhere that they had moved sensitive projects to another location. As to how the beliefs got started, I guess people would see lights and aircraft in the sky that seemed too advanced to be from Earth, and assumed that the technology originated from somewhere off-planet.
There have certainly been sensitive projects developed there. If they involved 'alien technology'…well, I guess that is the million dollar question. You have to rely on someone like Bob Lazar, and I never knew what to think of him. Fascinating stuff to read, regardless of if it is true or not.