The underdark
Whilst many arcanists insist that The Underdark is simply another plane, possibly part of the elemental plane of earth, accessible through naturally occuring gates, the majority of the population claim to know that The Underdark is located on the Material Plane, and that its dark influence is something to be constantly vigilant against.
Regardless of the nature of this vast maze of caves, caverns and tunnels, it is nonetheless a threat to any settlement unlucky enough to be located near a hidden entrance.
Possible Entrances to the Underdark
Keel Narrock Mines
Whilst the dwarves of Keel Narrock insist that their mines do not lead to the caverns of the underdark, it is undeniable that they seem to have a near endless supply of rare materials found there. These materials include extremely potent alchemical ingredients, rare metals and gems, and all manner of chitin.
Outsiders who have been granted trade approval with Keel Narrock are often able to buy these materials, albeit for a great price, however when asked about the source of the items traders always seem to imply that they simply traded with another outsider.
The Church of Sorrows, Allelwyn
It is rumored that during the construction of The Church of Sorrows, elves digging the crypts struck a hole that seemed to stretch down into the ground forever.
See Allelwyn.