Background: Real world
Undermountain is a dungeon situated underneath the city of Waterdeep, to be found the Forgotten Realms campaign setting for AD&D. It wasn't too big to start with. Well, not as big as it is now. It was the first dungeon published for the Realms, way back in 1975, and in 1991 there was a boxed set called The Ruins of Undermountain, containing the main areas of the first three floors. A second set called The Ruins of Undermountain II: The Deep Levels contains the next three or four levels worth. This is not a marketing ploy to sell more modules - this dungeon is BIG.
Background: Game
Probably what you came here for, right?
There was once a powerful wizard named Halaster, a legendary figure said to have begun many traditions among wizards and to have invented some of the spell processes still used today. He travelled off to gain some solitude for his magical research, as mages are wont to do. He travelled with seven apprentices to Mount Waterdeep, at the time a desolate place, and summoned extraplanar creatures to build him a tower circled by a wall, within which were fields and gardens. The apprentices tended the fields and studied in their own towers just inside the wall, seldom seeing their master.
Cutting himself off from human contact and dealing so often with strange creatures was not too good for Halaster. He started to get what we would call "weird". He had his summoned servants dig tunnels and store rooms and laboratories underneath his tower, a work which continued for decades. Eventually they broke through into the halls of a vanished Dwarf clan. This underground realm was overrun by the Drow, who Halaster exterminated, claiming this area as his own.
Halaster now took to living underground, and when his apprentices examined his tower they found it to be full of traps and enticing messages, luring them below. One by one they ventured down there, only to find monsters and traps left by their now mad master to defend himself from intruders. After two less able apprentices died he appeared to the rest and asked them to join him there and aid the defense of his home.
Halaster moved ever deeper into the mountain, leaving the upper floors as an amusing run of traps as he went deeper beneath the earth. He started to collect unusual monsters from unknown places using gate spells, and left them roaming the halls. His apprentices took areas of the deeper levels as their own private strongholds and worked on their own spells, most going as mad as, if not madder than, Halaster.
Today the dungeon lies beneath the largest city in the land and is often entered. The way in is no secret, one of the first adventurers to trek through the dungeon returned to the surface and tore down the remains of the tower, building an inn on top of it. The pit that people descend sits in the main taproom, with a waist-height wall. The landlord makes a nice sideline profit by charging adventurers one gold peice per head to decend into the depths of Undermountain.
Not many come back.
Ways in, Ways out
Besides the main pit in The Yawning Portal there are many ways into the dungeon. Halaster dosn't mind people having easy access to his dungeon, it just means he gets to watch more people die.
There are many ways in through the sewers of Waterdeep, though most of these are controlled by various groups - usually criminal ones. There are two known staircases, one controlled by the city guard and the other by a slave trader.
On top of the more commonplace ways in and out there are magical methods of entry: Halaster himself seems to enjoy putting gate spells in far-off reaches of the Realms to bring explorers from far away in to his dungeon, and most notable of all is the mirror in Myth Drannor which brings people to random places in Undermountain, linking two of the most famous locations for adventure together.
Traps and Hazzards
Or: Ways to kill parties off fast
Among the more conventional role playing game problems of large quantities of monsters trying to kill the heroes off its best to watch out for:
Unexpected Teleporters: These sometimes send you somewhere you don't want to be. Oh, and don't get any funny ideas about teleporting out of the dungeon, Halaster isn't stupid. Only his spells can teleport people around the dungeon. You were so keen to get in there and want to leave already? Tsk Tsk.
Halster's Apprentices: Don't mess with them. They are tough, and all are unhinged, appart from Trobriand (Though messing with him usually results in him finding where you live and sending some iron golems around to knock your house down in the night). Other notable people to run away from are Arcturia, who enjoys polymorphing people she dislikes, and Muiral, who also tried to improve his form as Arcturia did, except he failed and now lives as half human, half spider (If you've seen The Mummy Returns you know exactly what to expect).
Traps: But not just any traps. Some of the ones the writers pre-built are downright evil. Trapped in a sealed room with rising water? Nah, too easy. Lets throw in a bunch of fishmen, a psychic fish and some marine ghouls. A trapped door? No simple little poisoned needle or rock over the door. Trapped doors in undermountain start out mean and get meaner, culminating in a door with a wraith trapped inside. Direct contact, as usual, drains a level. Wear gloves or let someone else open the doors.
Game Structure
The actual playing of Undermountain is a big job. The Dungeon Master has a huge task just getting the whole thing ready, as the maps are largely left empty and waiting to be stocked. In the first boxed set the first map is roughly 1/4 full, the lower levels are even more empty than this. I managed to get just over 80 rooms on the first floor filled in with encounters, and I started the game with most of the dungeon left empty. After two or three sessions the party was about five rooms away from an empty area, so I could just have kept going with the direction they seemed to be taking.
Rather than running an Undermountain campaign as a straight dungeon crawl it's probably better to use it as a setting for many adventures. You can have someone go missing from the city, and hey! What do you know! They happen to be in Undermountain.... I guess someone should go down there and get them back. A thief can steal something from a party member, and guess where they hide out? Thats right....
On top of these little encounters the party could get involved with the more powerful groups already written into the campaign setting, such as the beholder who runs a slave trade on the third floor, near Skullport. Did I mention there's a whole city under there? With a huge winch for raising ships from the Sargauth to the sea above? "What's the Sargauth?" you say? It's a big river, deep under Waterdeep. I told you this dungeon was big.... You have a lot of possibilities, so go crazy.
It's what Halaster would want....