Half-Life

Screenshots:

Background:
Deep under the ground of a remote US desert scientists are experimenting on creatures of unknown origin. You are Gordon Freeman, one of those scientists, and during a routine day at work something goes very wrong. A time rift develops, allowing aliens to slip into your reality. When you regain consciousness the facility is in ruins, aliens are everywhere and to make things worse, the government is sending the Marines in to clean up whats left - including any survivors.