Ive been researching and looking into this stuff recently and it looks pretty serious. There is a lot of people who came out and talked about this place, most of them were killed, so you know something is up. The main base is called "Dulce Base" near Dulce, New Mexico. Supposedly there are branches of it that go throughout all of New Mexico, and North America. We are talking thousands of miles of underground tunnels/roads. It was started with a giant natural cavern system over a thousand years ago and they have been continuously adding onto it ever since, especially after the 1940s.
There are entrances/exits in every state in the USA. Supposedly the American Government has a lot of people down there working with the aliens. There are also many different species of aliens down there, everything from the small greys to reptilians, extra-terrestrials to highly intelligent evolved species of ancient dinosaurs. I dont know if all of this is true, but it looks pretty official from the stuff I saw. This is no joke either, there have been hundreds, if not thousands, of people injured and killed down there. Also, people reported missing have been seen down there never to be seen on the surface again. Seriously you need to look into this stuff, its amazing.
Here are some of the links I found:
(This is a very long read, but very very informative):
A Dulce Base Security Officer Speaks Out
(This guy, Phil Schneider, that worked near Dulce Base and nearly lost his life there in a battle with aliens, he was killed shortly after he came out and talked about Dulce):
YouTube - The man that killed 2 alien greys
(This seems like a good site, I havent looked into it yet):
Dulce Underground Alien Human Base
(This shows officical reports of underground bases and related in nearly every state in the USA):
UFO Bases, Underground Tunnels and Cities, Hollow Earth, Secret Government Bases, Alien Bases