Surturs Eyebrow is a lava river that crosses much of the Underdark sector Tiyaphis. It has enough rivulets coming off it that some consider it a natural maze. Dead lava tubes, those that no longer have lava in them, are scattered all along the river and in areas. Generally, the main flow flow, or artery is considered Surturs Eyebrow with the side rivulets and tubes being other names or having none at all.
In the Varelay Upheaval (2608 LE - 16 HE), mining by topsiders caused an enormous collapse along the main artery. This culminated in violent volcanic eruptions across Tha'lith. The pressure from the sealed off flow was forced down smaller tubes causing Hen'khas's volcanoes to become very active. The furtherest volcano, Zathess rapidly came about during this upheaval. The pressure going into Zathess has never returned to the time before the Varelay Upheaval so the land above still suffers. In 16 HE, four centuries after the collapse, lava and pressure finally broke through enough where the volcanoes waned in activity.
Surturs Eyebrow feeds the volcanoes of Hen'khas's and is fed by the lava run-off of these same volcanoes. On the surface, in areas where the elevation was not suitable for natural drainage, canals were built to channel lava into Malephar's rifts. Some of these canals are very old dating as far back as Varelay when slave forces in the tens of thousands were put to work building them. Today, the same canals and new ones are built and maintained by workers out of Goth-Dyvermoir. The canals end in cascades with lava descending into Dhark Bolg where they end the circle with their joining back with Surturs Eyebrow.