Underdark
“The Underdark” is the general term used to describe the vast array of tunnels and caverns that run beneath Lunistria. This network is so large and complicated that a modern geologist would seriously wonder why the surface world hasn't collapsed by several hundred meters. The most accepted explanation is that the Tunnel God was very busy early on while his brothers and sisters were distracted by all the consternation the birth of the Jungle god kicked up. In any case, it is entirely possible that the Underdark has a land area easily equal to that of the continental surface.
There are a number of known Underdark communities: Dwarves and Mountain Elves are the obvious examples, but it is entirely possible to find towns, cities, and even small nations of nearly every race down there. Humans turn out to be the bizarre exception: perhaps they require routine watering by the Storm goddess, lest they wither. (The fabled cavern city of al'Sofarre in the central Eastfyre Desert being an exception.)