No one has heard anything out of this sector for weeks. A team was even sent in to re-establish contact, only they never reported in. Something has happened and your force has been selected to work out exactly what. As you arrive it soon becomes quite clear what you are in for... There has been an infestation lurking underneath the ground, and they are coming out now.
It's not a question of if you can survive, but how long can you hold out for?