Undersigil

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. Gurincraag is a dwarven neighborhood in the Lower Ward; the majority of it is underground, extending into chambers of great majesty. Gurincraag is a secret, though.
The homes of the Dabus are also supposed to be close to the surface, but no one's ever found them.
The Twelve Factols Inn beneath The Lady's Ward also has an entrance to the Realm Below.
The Nest is a small settlement of ratfolk and wererats peacfully living together in the catacombs beneath the Lower Ward.
Beneath that is 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.

Alternative Name(s)

down below, the Catacombs, the Realm Below, the Labyrinths

Type

Underground / Subterranean

Location under

Sigil, City of Doors

Included Locations

• Secret Shrine to Aoskar
• Undersigil Sewers

Related Reports (Primary)

• Episode 25: I Think I Smell A Rat

Sources

Faction War
Planescape: Torment