Sometimes called the "Lands Below", "Underworld" (which often is confused with the regions of the Plane of Shadows of the same name) or "Underdark", the most commonly accepted term among scholars for the region below the surface of Rothea and above the molten magma at its core is "Terra Profunda". Below the continents, Terra Profunda is usually between 20 and 30 miles deep, although there can be considerable regional variation.
Much of Terra Profunda is riddles with caves and tunnels. While many of them are natural, others are the product of gigantic burrowing creatures, most prominently fire worms. This has created large areas habitable by living creatures, some of them sapient. They make use of ambient sources of heat or magical energies as well as emanations from the many natural portals to Faerie or the Plane of Shadows to survive.
Still, despite the occasional connections, most regions in Terra Profunda are fairly isolated from each other, and its inhabitants are fairly xenophobic, ready to aggressively defend the few resources they have.