Krephus Sink is a massive bore hole that begins in the desert Chen'ryon. It cuts through the surface and then into the bowels of Goulnacus and then across many other Underdark regions. This hole, like the desert Chen'ryon, were created by a primordial worm named Krephus. In the Dawn War, this colossal purple worm bored through the Web of Magic, allowing her to escape the Step of Foundation and cross-over to Bal-Kriav. She dug into the ground leaving a ridged shaft which now serves as roads between those from dark reaches and those on the surface. As she went down, she released her young purple worms into the Underdark. While she burrowed, sand, stone, iron, silver and anything else in the way of her maw were devoured. This left a trail of debris in her wake that at first spread up and outward creating the desert Chen'ryon. After some depth the blow-out from her sphincter went up and then fell down where it slowly filled the area behind her with a debris field that fills the bottom 50' of a 200' diameter shaft. At a depth of -10,000' the shaft bends bends east and then continues on a flat plane, northward for thousands of miles. Somewhere under the continent Amachan the Krephus Sink bends west and then gradually ascends until it is -5OOO' when under Gumar. In this area it zig-zags for a hundred miles and then ends where Krephus died of exhaustion.
At five miles under the surface, the shaft makes a turn, with much of it covered in sand. It turns east and heads far away - going under oceans and continents. The passage is gravel and sand, packed from innumerable things that have come to use it as a road. In areas where she encountered water, elemental rivers of earth, and lava, Krephus went over or under it.
Our flight from Jurusalax took us through areas of loose sand that were like quick sand, and past smaller passages that were made or being actively dug by her offspring.
I returned to Krephus Sink many years later and found that the purple worms were now using it as a breeding area. The locales said that they come back every year, some traveling great distances, not hesitating to attack those barring their way.