The Underground is the area of Earth that contained the Monsters for hundreds of years. The Underground is surrounded by a magical Barrier placed by the Humans.
The Underground is housed within an artificial mountain called Mount Ebott. Because Mount Ebott was created with a diverse amount of magics, the environment of The Underground is very diverse. It can be below freezing temperatures at one end, and extremely hot at another. There are five main environmental sections of The Underground: Two temperate locations, an area in Permafrost, a bog-like cave system, and a volcanic mountain.
There are two major rivers in the Underground. One river, known as the Acheron River, circulates around the perimeter of the Underground. The other river is the Phlegethon River, which circulates around and in the base of Mount Hot.