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

Ruins of Undermountain box set (TSR, 1991)

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See Also: 

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

Undermountain, session 2

Undermountain, session 3

Undermountain session 4