You're walking through the halls of a colonial outpost. It's obvious something bad has happened here. Sliding doors hang slightly askew, lights flicker ominously, and everywhere you see holes where something very nasty has eaten through the metal floor. And the floor below. And the floor below that.
You're a rookie in the Colonial Marines, sent to investigate a distress call by a group of colonists stationed on this far-off planet. You're scared, sure, but you've got teammates beside you to back you up. Working your way up multiple levels of deserted base, you end up at a makeshift barricade manned by another Marine. She's obviously as nervous as you are, but your motion sensor is reporting nothing but its reassuring neutral ping.