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.
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.