What about the Underdark? Is there a way to the Mantle of the Planet? Does Toril have a Mantle, a Core and tectonic plates? If not, how do volcanoes work? Yes, yes, yes, and yes (all covered in published Realmslore). Magma forces its way up and warms the Underdark, providing heat and light and a rich stew of minerals and gases. You're apt to get rather melted at the mantle, though. ;}#Realmslore

— Ed Greenwood (@TheEdVerse) May 12, 2021