The Underdark

"A world beneath the world. As varied and diverse as the world above." 

Geography

The Underdark is a massive world-spanning network of tunnels, caves and underground rivers. In places is reaches tens of kilometers deep into Adar's crust. Due to its vast size and complexity this underground system includes a wide variety of environments. There are huge caves which contain their own biomes and weather systems. Huge rivers on which ships might sail, leading to massive underground lakes. Yet most of the underdark consist of smaller caves and tunnels. Most natural, but many made by sapients of various kinds. 
The Underdark is one interconnected system. But many passages are difficult to access or blocked. As such the underdark is generally seen to be divided into several isolated sections. 
Underdark residents generally tend to regard the surface world ("topside") as a dangerous and sinister place, good only for its resources of slaves and exotic goods. 
The underdark is often classified into four "Layers". 
Upperdark Those layers closest to the surface. Its inhabitants are often seen as savages and near traitors, who prefer the surface world to the Underdark. These lands are far closer to the surface empires than to the Deep Empires of the below-world. This layers is sometimes refered to as the "frozen world", since it is so far from the heat of Adar's mantle. 
The Layers These layers of the underdark are the "wilderness" of the underdark. Most of the Layers are natural tunnels, inhabited by various species native to the darkness. Exiles from the deep depths often come here, hoping to one day be able to return to their homes. This layer of the underdark is also the furthest of which the surface world is familiar since explorers rarely survive it, or find the hidden passages leading deeper down. 
Warmlands Go deep enough into the underdark and you will eventually come to tunnels that are naturally heated by the mantle, such that they are always pleasantly warm. Here the great civilizations of the underdark flourish and fight. Drow, Ilithid and Duergar all build cities and small empires in these depths. 
Firelands Deepest of all are the firelands. Here the heat can sometimes be fierce enough to kill, and tunnels move and collapse and are re-formed frequently. This is a place few go willingly, and the creatures here are either hostile to most other life, or insane and unpredictable.