Greys are arguably one of the most iconic Aliens on earth.
They are capable of interstellar travel and other crap like that using
their spaceships. On 01/33/70 scientist in Roswell, Old Mexico, captured an injured Grey. It was down along with its space ship.
Some theories suggest that aliens are actually humans in the future and that we will eventually evolve in to Greys.
This is a very good theory that has a ton of sources that could back it up