Gamesmaster's Guide

From the introduction: The world is a dome upon which the lands of the dwarf, the elf, the hobbit and the human co-exist. The great seas pour outward, toward the fringe of the dome. They cascade into the darkness, only to be sucked back into the hidden world of Undersea, a second dome beneath the first, there to travel back toward the hub and the great world-forging Weight Sphere of Belril.