Aliens have been on the Earth for thousands of year. How they got here I haven’t been able to figure out yet. I’m sure it will come out during the next couple of weeks.
Alex Collier seems to be the guy that’s the most knowledgeable about aliens on Earth. Here’s a video of him talking about them (from 1994) and if you want more, there’s an entire page of videos here. I will let you be the judge of whether he’s credible. The problem is that there’s so far no proof, not from Alex, at least.
There are, however, lots of folk that swear that they exist. In fact the Med Bed technology has supposedly been given us from aliens. I’m certain more is going to come out about this over the coming two weeks.
Here’s an article on a number of well-known people, including President Eisenhower, who have stated that they do indeed exist here on Earth.