No native of the Cage would ever call it UnderSigil; calling it that is a good way to be marked as Clueless. Real Cagers just call it "down below," "the Catacombs", "the Realm Below", or "the Labyrinths" (but never the Mazes).
Closest to the surface are the sewers, catacombs, and dungeons excavated by surface dwellers, as well as neighborhoods that have been built over and buried by Sigil's expansion. The homes of the Dabus are also supposed to be close to the surface, but no one's ever found them.
Beneath that is the territory of the wererats, the Warrens of Thought where the Cranium Rat hiveminds rule, drowned nations, nations of undead, and stranger things. The Darkers are a culture of people of all kinds who live beneath Sigil without any light at all. Whether they're a Faction or just a subculture is unclear. They are criminals, madmen, and people with just no other place to go. There are also said to be tribes of living ghouls (see Proto-Ghoul), and cagequakes are a constant threat.
No one knows how far "down" the Realm Below extends. Nobody's ever dug so deep that they come to the "other side" of Sigil's toroidal surface. Those who dig deep enough simply disappear, presumedly stumbling into a random plane, and nobody's ever been able to figure out a pattern to it.
Player Notes Edit
Undersigil is actually a collection of multiple interconnected adventuring areas for low- to mid-level characters. Each area has its own set of enemies and challenges.
See the following breakdown by area:
Undersigil 1- Sewers
Undersigil 2- Goblin Tunnels
Undersigil 3- Necromancer's Lair
Undersigil 4- Mind Flayer Lair
Undersigil 5- Sewer Outflow
Undersigil 6- Basilisk Lair
Undersigil 7- Humanoid Tribes
Undersigil 8- Spider Ruins
Undersigil Lake- Undead Pirates