This twisting maze of pitch-black tunnels, half-carved corridors, caves and dead-ends lies far beneath the surface of Thera. Home to svirfnebli, dark-elves, duergar and other creatures too horrific to mention, death lurks around every corner, waiting to claim the unwary. Entrances to the Underdark from the surface world are numerous; exits, however, are quite rare, for their existance remains the closely guarded secret of those who create them.