The Firelands is the appropriate name given to the region of the Underdark that lies beneath the Chultan Peninsula. It is a region of high volcanic activity, with near permanent lava flows, chasms, and numerous portals to the Plane of Elemental Fire.
Burning Rift: The Burning Rift is the largest volcanic rift in all of Faerun, a titanic chasm stretching for 500 miles from the Kobold Mountains to the Sanrach Mountains (passing directly beneath the Peaks of Flame), and 20 miles at its widest point.
The Burning Rift is the heart of the Firelands, which exists only as a series of lava tubes and caverns created by the massive volcanic activity from the Burning Rift.
The Burning Rift is riddled with portals to the Plane of Elemental Fire, and is in most places far too hot for living creatures to exist without some sort of resistance to heat and flame. The Burning Rift is home to tribes of salamanders, firenewts and other heat loving creatures or those from the Plane of Elemental Fire. There appears to be no single leader among the fire creatures of the Burning Rift, and groups of them war with each other over the best access to the rift.
Sages speculate that the Burning Rift was caused by a falling star that cracked the ground apart when it crashed into the Jungles of Chult. The path of this falling star can still be seen, rolling and bouncing from the Kobold Mountains to the Peaks of Flame (through the Valley of Lost Honour), with remnants of the meteor continuing on into the Chultengar.
Elementals: The Burning Rift, with its planar tears leading to the Plane of Elemental Fire, is home to a variety of creatures made of elemental fire. Unusually for creatures made of elemental energy, these creatures seem to exult in torture, causing pain and fear, and incinerating living creatures, in short they are viewed as evil by sentient beings.
Fire elementals, lava elementals, salamanders, efreet, and azers are all known to thrive in the Burning Rift and a number have made their way into the lava tubes and caverns that form the rest of the Firelands.