The Deepways, known to some as the "Underdark", were an enormous system of subterranean caves, caverns, and tunnels located beneath the surface of the world. Like many natural formations on the Prime Material, these tunnels had counterparts in the Vivid Realms and the Umbral Realms. Given their size and scope, it was not always possible to travel from one end of the Deepways to the other; it was divided into several domains that were similar to the landmasses of the world above. Some areas were always changing due to the shifting of underground magma rivers or the emptying of underground lakes from earthquakes cracking the rock; this made that the scant few maps of the Deepways were themselves highly unreliable.
The Deepway's underground landscape varied from small subterranean fissures, almost impossible for Mortalborn to crawl through, to more conventional caves and caverns. There were also wide expanses of deep water in which coral caves grew, Deepshimmer emnated, blocks of ice that creatures hollowed out and dwelt inside, and cave-like spaces within masses of fungus, bone, or bubbles of magical force. This varied landscape was as dynamic as the surface of the Head of the Greatwyrm.
Denizens of the Deepways referred to the surface lands as the "World Above" or "Upper Worlds", and categorized their realms below it into three divisions: the Upperways, the Middleways, and the Lowerways. For a cave or cavern to be considered part of the Deepways, it had to connect to one of these realms in some capacity.