Underworld

The Underworld is the antithesis of the Surface World, and where the realms of the gods, infernals and fey are located.

It is widely believed to have been originally created by the Undergod.

It's features however, are confusing to the mortal mind, and often contradict common logic. For example, some explorers claim that by delving deep enough into the Underdark, they have reached the Underworld, meaning it physically exists under the Surface world. At the same time, however, it is many times (perhaps infinitely) bigger than the Surface World, with the Feywild alone (as a reflection of the Surface World) being at least as big as its reflection.