A vault in the game Fallout 3, probably the creepiest one. Although you don't find out at first (you must hack a computer to figure it out), the air has been infected, most likely from the nuclear explosion that started F3. The residents attack you and each other, and you have visions of people who were important in your life at Vault 101.

When you eat dirt for the first time and leave your family to build a shack in the woods