Abyssal Planes

The Abyssal Planes are a plane of existence associated with chaotic evil. The number of layers is endless and neither the gods nor its residents do not know the true extent of the infinite nature of the Abyss.

From The DMG:

The Abyss embodies all that is perverse, gruesome, and chaotic. Its virtually endless layers spiral downward into ever more appalling forms. Each layer of the Abyss boasts its own horrific environment. Although no two layers are alike, they are all harsh and inhospitable. Each layer also reflects the entropic nature of the Abyss. In fact, much of what one sees or touches on the plane seems to be in a decaying, crumbling, or corroded state.

Known Layers:

Lower Ilizzil, the 546th layer. 
The Crystaline Mausoleum, the 77th layer.