I doubt the Mapper will ever tell you 'cuz it's more fun to make theories than outright say what's going on.

So, I myself think that...What happened is, there was simply a Research Site that stumbled itself into a very old Nordic Tomb. Said Tomb had a Draugr (or even Draugir, as in more than one) that was annoyed by their presence, so he starts a massacre pretty much.
The research team tries to hold, they even had some soldiers with them, but sadly it just didn't help them.

I have no idea why you are even in here, from what I can see, you're probably one of the team trying to escape it, but the chopper crashed and everyone but you died, or some tried to get back and died in the way, so now it's up to you to get back there and find another way out, which considering that when you reach the Communications Tower you know exactly who to call, it further adds my theory that you play as one of the Research Team indeed.

The rest is just based on what Ending you've got, really.