The Deep is not a single cavern or unified city, but a vast subterranean world beneath the Frostkarn Range. It contains underground rivers, black lakes, mineral forests, crystal formations, fungal jungles, ancient ruins, and passages descending into regions that even the Dark Elves fear to explore. Great cities are built around dependable sources of water, valuable mineral deposits, defensible caverns, and sites believed to possess religious significance.
Travel between settlements is dangerous. Roads pass through monster-haunted tunnels, territories claimed by hostile peoples, and regions where ancient magic has distorted the stone. Dark Elf houses maintain fortified waystations and heavily armed caravans, while lesser travelers must purchase protection or risk enslavement, robbery, or death. Control of a major tunnel, bridge, underground harbor, or source of food can make a family wealthy enough to challenge the ruling houses.