From the publisher: Skalds’ repertoires are full of stories concerning Ertha’s Realm, also known as the Underearth and Erthaheim. Tales of a subterranean realm of vast caverns all interlinked by countless miles of tunnels, warnings of a race of exiled elves who seek nothing more than total domination of the surface world, and yarns involving great cities with populations in the thousands, echo around inns and drinking halls the length and breadth of Rassilon. This supplement explores the truth and opens up underground exploration beyond the confines of the "dungeon." As such, it is less a guide to specific locations than a survival manual for players and GMs alike. This supplement contains rules for light and sound, movement, rope use, and tight squeezes while spelunking, details the minor god of underground exploration and the alien deity of the fungals, one new hazard, and 8 subterranean monsters.