Geology and Environments

A cave is a natural opening in rock that is large enough for creatures to enter. The Underdark, simply put, consists of a linked network of several massive cave systems. Large portions of it do fit the definition of “a natural opening in rock,” but the Underdark also encompasses areas of deep water that hide coral caves, hollowed-out sections of ice in which creatures live, and places where fungus, bone, or even pure force form caverns.

Underdark terrain is dynamic and changeable. A map drawn a few decades ago may show tunnels that have long since collapsed, or lakes that are now dry. Terrain can change gradually over the course of decades, centuries, and millennia, or swiftly as a result of an earthquake or volcanic eruption.

TOPOGRAPHY

The surface world of Tellene is marked by mighty mountains, high plateaus, and vast plains. The Underdark possesses none of these features, but it does have physical features all its own. Unlike the surface world, the Underdark is uniquely three-dimensional. Knowing the direction of true north is not enough to navigate the land below; a traveler must also know the depth underground of her destination. It is possible to find the correct coordinates but still be several miles too deep or too shallow.

Many of the Underdark’s features can be explained as nothing more than the results of purely physical forces, although sometimes on a grand scale. Other features would be unlikely or even impossible in the world above where magic, elementals, and the divine have shaped.