From Halopedia, the Halo wiki
Perdition takes place in a thermonuclear reactor facility in Pilvros which is currently undergoing a catastrophic meltdown. A monorail passes through every so often due to the evacuation of the city, with multiple abandoned vehicles such as several tractor units and one or two forklifts left on the street. It is asymmetrical, with the melting-down reactor at one end and an enclosed facility at the other. Side hallways cut through the walls, and in the middle of the map is the former control room for the reactor, which serves as a hub for close-quarters infantry fighting in most game types.