The Underdark is one of many names for an extensive series of caves, tunnels, and underground chambers that riddle the crust of Tol. These caves were formed by the Primordials during the creation of the world, through means such as water action, lava tubes, erosion, and the like. They continue to grow and to be expanded by creatures that made their home there. It’s known that at least one vast underground sea lies within the Underdark, as well as lakes, rivers, and cities.
The most commonly encountered dwellers of the Underdark from the perspective of the surface-dwellers of Tol are the Drow or Dark Elves. These creatures were forced into the dark places of the world during the Feywar by the Eladrin and the Elves. There are several cities of drow in the Underdark, but they are dominated by worship of Lolth, their foul goddess.
There is at least one gate to the Abyss within the depths of the Underdark, and demons are known to roam there from time to time. They make alliances with the drow and other, fouler creatures, such as aberrations. The foul aberrations of the Far Realm find the Underdark convenient to their purposes, as those creatures that most often oppose them do not venture there lightly. As a result, cities of aboleths, beholders, grell, mind flayers, and even more inscrutable beings can be found there.