Undermountain

The great dungeon of Undermountain, also known as the Halls of the Mad Mage, is a vast subterranean complex carved into the roots of Mount Waterdeep and the great plateau on which the City of Splendors sits. Although Undermountain extends underneath the entire city and even beneath the sea floor to the west, the bulk of the dungeon lies beneath Mount Waterdeep (hence the name), as well as the western reaches of Castle Ward and Dock Ward.