From the back of the book:
It started only days after the invasion. Deep in the planet's interior, hidden from prying eyes, the caverns were created.
The caverns went down for miles, reaching to every corner of the Earth. Populated by creatures both kind and cruel, the tunnels grabbed Storm Knights, pulling them into an underground realm of death.
The wonders of the underground world seemed endless: New civilizations, incredible treasures, and a dangerous passageway to a fringe reality. Only the bravest will survive the perils of the Land Below.
This supplement is designed as a companion to Torg: Roleplaying the Possibility Wars. It introduces players and gamemasters to the mysterious tunnel network running throughout the Earth, and provides complete information for a new fringe reality connected to the Earth.