"We are not who we are."
meets The Thing (1982)
, without all the cool stop-motion effects (though still maintaining a lot of the suspense). In essence, a team of geologists studying Arctic rocks lapse gradually into insanity and kill each other off one by one. Mulder and Scully, along with two scientists, a doctor and a pilot, fly over to the base to see what's up. Unsurprisingly, the craziness starts affecting them too.