An adventurer could spend a lifetime down there.
…however long or brief that may be..

The Undermountain box set is perhaps the largest dungeon TSR ever released. Definitely the largest if you include Undermountain 2 and the other expansions.

The Ruins of Undermountain box set came with a number of components, but this post will focus on the maps.

Undermountain has been criticized since release because most of the rooms are “empty”, unkeyed. This is either a feature or a flaw, depending on your DMing style. There is a playstyle in the OSR I call stock-on-the-go, filling in dungeon rooms as the party approaches, with imagination and/or well-written random dungeon dressing and monster tables. I’ve done this myself, when I ran my boys through Undermountain (links at bottom of post).

Ruins of Undermountain box set (TSR, 1991)

1991, 1993 … TSR 1060 … ISBN 1560760613

Contains: 128-page perfect-bound book, 32-page book w/cover, 4 fold-out maps, 8 Monstrous Compendium pages, 8 cardstock accessory sheets.

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Level 1 (TSR 1060XXX0701)

Level 1 has the iconic entry well far beneath the Yawning Portal.

Level 2 (TSR 1060XXX0702)

Level 2 has a number of ziggurats and raised platforms.

Level 3 North (TSR 1060XXX0703)

Level 3 North is dominated by an underground river.

Level 3 South (TSR 1060XXX0704)

Level 3 South features Skullport

Both Level 3 maps connect of course. The river continues south, and there are a couple of small dungeon passages that pass through.

Another sign of Undermountain 1’s long print run: Close-up of two different lvl 1 maps.

When going through my parts in the Boneyard, I noticed that one set of UM maps had a glossy texture. Gloss is hard to pick up on camera, but a flash reveals the sheen. So different print runs of the poster maps.

See Also: 

Ruins of Undermountain 2: Go Deeper in Forgotten Realms’ Megadungeon (1994)

Undermountain, session 2

Undermountain, session 3

Undermountain session 4