The Great Fissure is a massive crack in the rocks deep below Tethir. The fissure runs for roughly 150 kilometers, and is about 3 km tall. The fissure varies in width from its narrowest point (the Fellglow bridge), only 75 meters across, to its widest, more than 2 km. At the top, the walls of the fissure come together, but at the bottom they spread out and drop into an enormous lava lake. The lava lake rises and falls up to 500 meters with the internal pressures of Anis. These movements are referred to by the Dwarves as “tides”.
The fissure is an essential part of the Deep Dwarf Defense Network (Triple D), since it presents an impassible barrier even to enemies that can drill through solid rock.