There are probably quite a few places where the US government carry out secret tests of the military kind and some of these don't really "exist" because they are officially designated as such ie. officially not "existent".
This sort of secrecy is a leftover from the cold war era but is still observed in many military establishments.
As for the aliens that are supposedly kept there, I think we have to answer a more fundamental question - Why would aliens (smart enough to space travel to earth) be content with cutting up cows, following cars on the highway and scaring rural folk, when they could land on the White House lawn and meet the President?
At the dawn of the radio era we thought that aliens would get in touch
with us by radio. These days I suppose we hope they'll send us an email?