Beneath the surface region Troll Bogs is its underside, a low-light realm often considered just as wet and swampy as the land above it. This Underdark region is called Slilp-Urp. It gets so much ground seepage from the wetlands above that in some areas it is like raining underground. Only a small fraction of all this water makes it to the Pearl Sea, for the rest drains into deeper tiers. All of this is by design. Slilp-Urp has a latticework of porous rock which serve as funnels, draining the water even deeper. These seams of rock were specially made for this region. The Prime Architect helped Silvanus create a system to drain all the excess water coming out of the Radullu Rift. This help by the Prime Architect would be his death sentence.
In the Horgon Era, the city-state Nagdúrzol used their alien technology to blast away the old lava tubes of the Orngrek to the river Sadregnen. These were then flooded, giving Nagdúrzol access to the Pearl Sea. One of the easiest ways to reach this Slilp-Urp is through the Tûnin-Gil sea caves. Today, these caves widened and expanded by the Nagdúrzol, allow sailing ships to leave the open skies and enter the darkness shrouded lands of Slilp-Urp.