(This is for the fanfiction "Crossover Together". Info may change from canon information)
Contrary to popular opinion, the Underworld is highly diverse in landscape much like the Surface World, with numerous landscapes and landmarks lacking in the Overworld. These include lands such as Tartarus, and The Great Rift.
As the realm is underground, it is naturally cooler and dryer than the other main realms, although large bodies of water can be found in the realm (ex. Styx River). Due to the magic present and preserved in the Underworld, the skies during the day hold a similar effect to the Northern Lights of the surface world.