In Eleg's mantle there are a number caves, small enough that modern seismological sensors can barely detect them but large enough to house entire cities. Little has been explained about how the area is habitable, but they are known to have a lot of technology specifically designed to protect them from the heat. According to Voreese, both Sermung and Sai-hee spend much of their time in the undercrust, and each of them and their strongest supporters projects a large safe area, although the process this works by is unknown.
According to Garovel, the Undercrust has a population of about 500 million people, although it is unknown how accurate his information was given that he hadn't been to the Undercrust in about 300 years.
Connection with the Surface Edit
It is very difficult for normal humans and most servants to travel between the surface and the Undercrust. Few servants have powers that would let them effortlessly penetrate the sheer amount of rock between them, and any servant will also need to have enough passive Soul Defense to resist the extreme temperatures. There are a number of places where tunnels exist between the surface and the Undercrust, but the only one to have been revealed is the pit under Warrenhold, for which it was built where it was.