Undermountain

"Tales of this legendary dungeon below Waterdeep are told well by many in the city, but I’ll provide you with the basic truths here.

Beneath the plateau of Waterdeep lies the largest and deepest dungeon in the world. It sprawls out under the city, said to plunge as many as twenty levels deep. The Melairkyn dwarves first excavated the tunnels that would become Undermountain, and the drow are said to have dug their own tunnels up from below. All were claimed, altered, and expanded by the mad wizard Halaster and his apprentices — who are believed to dwell in the dungeons to this day. What drove them deep into the earth remains a mystery, but Undermountain’s allure is a siren song that still draws many. If you want to see adventurers descend into the depths, or perhaps glimpse some returning with wondrous treasures, visit the Yawning Portal in the Castle Ward."
-Volo