It was yet another Weyland-Yutani Corporation “science colony.” The fact that there were Colonial Marines everywhere doesn’t necessarily mean that it was a military installation because the USCM are mercenaries paid by Weyland-Yutani to protect their assets in an environment known to be hostile.
The fact that there are gun-toting mercenaries everywhere is evidence that, as usual, the Company is up to no good and experimenting with xenomorphs. That would be why the Alien campaign starts you out as a facehugger whose egg was in a crate that just happened to get knocked over when the pilot failed to secure it properly before being called away.
It was a decent game, with well-balanced multiplayer and a well-written single player story. Monolith did a really good job of winding the three threads of the story into a coherent whole, which is something Rebellion didn’t really bother with in their second game (their first AvP game was actually “Alien vs Predator” on the Atari Jaguar, and the three single player campaigns were really well done in that one), who chose instead to focus on getting the ambiance right and using it to make a terrifying multiplayer game. The Lithtech game engine just wasn’t up to the task of creating both the dark, claustrophobic, “shit gonna jump out at you from nowhere” feel for the Alien and Marine campaigns and the dense, muggy, “hopping through the treetops hunting softmeat” feel for the Predator campaign. The indoor environments for that game were pretty well done, but the outdoor environments just didn’t have the right tone and gameplay feel.
I’m really looking forward to this one, and have been ever since I found out about it a few months ago. The only real drawback is that I’ll only be able to play it at night after my daughter is asleep, because it’ll be just a little scarier than I’m willing to expose her to. 
She digs Half-Life, Half-Life 2, and Bioshock, but AvP would give the poor kid nightmares (shit, it gives me nightmares!).
It’s pretty much universally agreed that the first AvP movie was a massive disappointment. I could go on for hours about where they went wrong, but really it boils down to setting. If you replace every reference to Earth and Antarctica in that movie with the name of yet another alien world that WYC is attempting to colonize, and remove that stupid bit about the Mayans and their pyramids, then the movie would work.
But if you’re going to turn those xenomorphs loose on our homeworld…that right there is the scariest shit I can think of. So you’d better get it right. One facehugger in a populated region can wreak a lot of havoc, and once those bugs start reproducing or looking for a way to set up a nest for that purpose…
AvP: Requiem got it right. That movie - especially the unrated version - was scary as shit. 
Anyway when BM will come everything (avp included) will dissapear from my head for “few” weeks