Phlegethos

From Fiendish Codex II: Tyrants of the Nine Hells (3.5e):

Searching for the flames of Hell? Look no farther than the layer of Phlegethos, where the air ripples with heat distortion, the lakes and rivers run with bubbling molten lava, and jets of flame spout from a million fissures in the red-hot, crystalline earth.  The plane's capitol city of Abriymoch is built inside an active volcano; its streets flowing freely with molten lava.

The fire of Phlegethos appears not only intelligent, but spiteful, too. Since devils are inherently protected from burns, its hungry tongues must seek out the more vulnerable flesh of mortals and other planar travelers.

From Manual of the Planes (3e):

The fourth layer of the Nine Hells is a place of fire and pain much like the Fire Plane. Rivers of liquid fire rule the land, and the air is consumed with dancing flames—in fact, the layer of Phlegethos has the fire-dominant trait. Creatures without fire resistance or immunity are soon immolated.  The Heroes of Hell found this out the hard way while they were Searching for Senek.