A large area of Tiyaphis is made up of old and active lava tubes. Most of these are dead tubes which serve as natural passages for the creatures of the Underdark. This area is called Sephu'phis, named by the master of Varelay after a maralith that served as a guard captain and concubine of the demon emperor Aperoq. The passage of lava through the area was shifted greatly during the Varelay Upheaval (2608 LE - 16 HE). This resulted in many of the lava tubes becoming inactive, or dead.
Sephu'phis is a maze of lava tubes that are both active and dead lava tubes. The active ones lead to lakes of lava or fungi-forests that thrive near the heart. The dwellers of the region claim that nobody has traversed even half of the passages.