The Dwarven Refuge Crisis

Mining deep was a way for the dwarves. The best metals, gems and Rock was the deepest. Cities were built in the depth by ancient Dwarves, and digging into the very mantle of the planet was common place, and understood the dangers. Little did the Dwarves of old know how dangerous it was. One day out of these tunnels, the Reavers emerged. Monstrous creatures, filled with hate, and ravenous for flesh and meat, but not sustained by it.

Revears are very large creatures, and they would overwhelm and devour city after city in the depths. Tunnels were filled, and battles were fought, but it was all in vain. As they encroached closer and closer to the surface, it was discovered they did not travel into the light, and the dwarfs knew, they had no option but to abandon the cities of there ancestors. Not all felt this way, some remained behind, but they were devoured. Scouting parties have revealed that all the Dwarven cities are overrun.
  The Dwarfs were met with prejudice and rejection in the beginning as they attempted to find there home on the surface, but as time went on, the Dwarfs had filtered themselves into the nations of the surface world. To this day, any cave that connects to the ancient Dwarven tunnels is extremely dangerous, though with the help of the Dwarves many of these have been filled in, though cave collapses could lead to any cave having Reavers within.
  Not all are ready to give up on there heritage forever, but beyond the light of day no solution had been found for the Reavers, and most simply are thankful that the Reavers did not follow the surface.