The Three Sisters were semi-active volcanoes. They were connected to a complex of lava and steam vests leading deep into the depths of Toril. (These same steam vents connected to the Dour Fissure, a deep gorge farther to the north.) So much heat was in the ground of the foothills of the Three Sisters that the small dale adjacent to the hills was on average 25° warmer than the surrounding valley. This increased temperature allowed for limited agriculture, which provided much of the food for the kingdom of Hartsvale.