To light a firepit, a curly root must be added to a dry patch, which can be found mainly in the parts of the cave connecting the main chambers. Once added, the root has to be lit using a tinderbox. This process gives 378.7 experience in Firemaking and requires level 83 Firemaking.
Once the firepit is lit the roots can be used on it.
Adding additional curly roots to the firepit grants an additional 378.7 experience per root. Each root also keeps the firepit lit for an additional 30 seconds.