The Center of the Earth is the abode of Nergal and his family. At the center of the planet, it features a dome of molten rock and a vast field of checkered blocks. In Son of Nergal it's also shown to have a castle, though we mostly see Nergal's family residing in a rocky cave.
Nergal's home as seen from the outside.
It can be accessed through a crater in Endsville's junkyard or the volcano located near Camp Paradice. While Nergal defines the center of the earth as a lonely place where you can't make friends, Aunt Sis is comfortable here. Nergal Jr. does not like it that much but appreciates his home.
It's a running gag for someone to break it's ceiling and flood Nergal's home with molten lava, with everyone being only mildly inconvenienced by it.