Volcanoes

It is not totally true in that descending 50 miles straight down in any spot brings a traveler to magma. Volcanic activity is extremely variable in the Underdark. Isolated pools of magma seep up almost to the surface in all sorts of places without any other volcanic activity, and in other places deep tunnels and vaults support humanoid settlements at depths where magma should be all that’s present. Whatever the cause of these surprising conditions, racing rivers of molten rock, caverns full of brimstone and sulfurous reek, and scalding geysers and hot springs can be found at almost any depth in the Underdark. Underdark volcanoes aren’t really mountains-they are usually tremendous fissures or magma chambers that can vomit deadly rivers of lava into nearby caverns with little or no warning.