This morning I compiled and synthesized more information cribbed from Planescape canon about the undercity. (Thanks to John and Chris W. for tips about where to look.)
Putting this together, I just couldn't resist the siren song anymore no matter how much candle wax I shoved into my ears. Following Micheal's lead, as of this morning two bright and shimmery gates were penciled into locales in the Hill Cantons for players to discover (or not) as a possible entry-point for field trips to that strange, cold place.
Features of UnderSigil:
The undercity definitely exhibits many of the reality-bending features of the Mythic Underworld, though of a more deliberately fabricated kind. Sigil's underground is not made of stone for example. The underground material is hard, you need a pick and shovel to tunnel through it but it crumbles like rice cake when removed from the underground. Since it's not really rock there are no natural caves or cavern systems.
Many of the passages are unstable, rare localized “cagequakes” will often bring down tunnels causing 3d4 damage (save vs. breath for only half) to anyone caught in such a collapse. Ventilation is bad, travellers can only take exerting themselves at half rate (presumably this is where the old school resting rules come in).
The complex is hundreds of square miles of tunnels and chambers, most of which are empty. The tunnels rarely get wider than 10 feet across with even narrower corridors common.
The upper levels are fairly close to the surface level and are mostly composed of sewers, crypts, and dungeons (the literal kind, the High Houses have deep and extensive ones for prisoners and slaves) . Dabus tunnels, were-rat passages, and vermin warrens connect all of these areas with passages extending to deeper levels. Darkers, mysterious exile communities living wholly without light, live in areas just off these branch-off tunnels.
There are many entrances to the Realm Below scattered through the city. The easiest entrance is through the sewers. You have to wade though the muck and filth for a few hours but you will find mysterious passages and doors leading off. These split-off passages lead downward at very steep angles (presumably long too).
Surface crypts will often lead down into the catacombs too. Cagers (Sigil dwellers) will often dig down underneath the crypts to bury their dead and will enlarge on existing catacombs when they find them. Ghouls who live in large numbers under the Hive and Lower Ward often dig up to the crypts too.
Many ruined surface structures, especially in the slums of the Hive, have entrances into the catacombs. The dwarven neighborhood, Gurincraag is rumored to have several entrances. A deep, 200-foot shaft (in the Lower Ward?) with filth-dripping walls leads straight down in the undercity. There is no visible means of descent but a maze of tunnels branches off in all directions at the base.
The rat-dominated realms have four separate—and vast—networks dominated by collective brain entities called the Four Great Minds. Sewer rats, cranium rats, and were-rats are part of these domains. Were-rat warrens are filled with traps—mostly of the sharp spikey, poisonous and blade persuasion. Entrances to these warrens mostly extend to the walls of the Ditch in the Lower Ward.
The Temple of Darkness is a major complex of sinister Eviiiil underground. It's a refuge for the scheming “yugoloths”, who have nothing to do with crappy Balkan-made cars but are a race of Neutral Evil planars we old schoolers knew and almost-loved as daemons. It's mapped in Faction War, making it one of the only major underground sites (to my knowledge) with a tabletop canonical map. But keep that under your hat this place is secret and hidden.