From The TimeSplitters Wiki

Bunker
Size	Medium
Location	Underground in an abandon military bunker
Climate	Dark/probably night
Unlockable	Unlocked by Default
Recommended Bots	N/A
Recommended Weapons	N/A

The bunker was introduced in TSFP and has a sort of dark and lonely feel to it. The main battle points are in tunnels with the tracks in them or the crapped hallways.