The Underdark: A World Below

Far beneath Raen'dor (and the rest of the Material Plane) lies the Underdark, a vast expanse of subterranean tunnels and massive cave systems. Several of the mortal races who live in the realms originated in the Underdark (drow, duergar and deep gnomes, to name a few), and many monstrous creatures (certainly not the least of which include the dreaded mind flayers) emerge from the depths to terrorize the surface world-- or if nothing else, make journeying into the Underdark extremely dangerous. The whole of the Underdark is extremely vast; even the most learned of scholars do not know how far down it truly extends, and it is theorized that the countless caves and tunnels link all continents across the Material Plane together.
While most people of the current age are aware of the Underdark's existence and the history that comes with it, the areas of the Underdark below Raen'dor are still to this day widely unexplored. Enough expeditions are planned the a few adventurers specialize specifically in traversing the Underdark, but these forays have rarely caught the attention of noble Houses that would sponsor explorations of a grander scale. Perhaps one day the mysteries of the Underdark will be brought to light (pun intended!) but for now it remains an alien world shrouded in darkness below.