Dwarves and Fortresses
Many of the scarier denizens of Mithia live underground. As there is no grand and pervasive network of tunnels catacombing the earth, a surprising number of them share a curious and magical ability. These creatures have the at-will ability to “swim” through dirt and unworked stone, leaving no mark of their passing and moving at speeds that often exceed their normal walking pace. Dwarves call the creatures with this ability burrowers.
Quite fortunately for dwarves, burrowers can only travel through materials that were arranged through natural processes. Stone that has been worked and smoothed by the efforts of sentient creatures is impassable to them. The walls of tunnels can be converted to such a state by the efforts of a mason (see Crafting for details), and so a carefully maintained underground structure is usually safe from the invasion of burrowers. But cave-ins and new excavations present a danger that wise dwarves are vigilant in guarding against.
The behaviors and habitat of burrowers vary widely by species, and the intelligent races with that ability are of course much less predictable. Some live primarily on the surface and burrow only to hasten travel. Some spend most of their time submerged in stone, hunting other burrowers. And some use this ability to travel among the many pockets of open space that pervade the underground realms like bubbles, building strange and labyrinthine civilizations in the darkness, isolated from the world by miles of stone.