Into the Deep

Vast subterranean worlds have become a big part of D&D but one that is not nearly as prevalent in the source fiction. There are occasionally small kingdoms or lost worlds under the earth, the token race of mole people or deeply dug mine. But these are small compared to the massive labyrinthine world of the D&D Underdark (or the Pathfinder equivalent of the Darklands). Tunnels and cavern systems that can stretch across continents large enough for entire cities or even seas.