Far below the surface of Ulskandar, beneath oceans and continents, there lies a vast, sprawling network of tunnels, caverns and caves known collectively as the Underdark. These underground spaces can vary in size from tunnels that a Gnome could barely squeeze through, to caverns that could house entire cities. It is thought that the nexus of the Underdark is continuous, with entrances leading down to its subterranean spaces appearing all across the world, but no-one from the surface has been able to conclusively prove this. For many of those that live on the surface, the Underdark is all but a myth, with stories of its denizens being used to frighten children into behaving at night, but for some those denizens are all too real. Some sapient species, Dwarves in particular, are much more keenly aware of the presence of the Underdark and its denizens. As the surface peoples that are the most comfortable dwelling beneath the ground, especially in The Union of Mishtoon, Dwarven communities often inhabit the peripheral space of the ‘near-surface’ between the surface proper and the Underdark. All Dwarves are told about the potential riches that lie, waiting to be discovered far beneath their towns and cities, but they are also told about the corrupting influence of the darkness below.
The different regions of the Underdark can vary wildly from one to another. Some areas can be desperately bleak and cold, with nothing but featureless rock as far as the eye can see, some regions are unbearably hot, with lava bubbling as rivers and lakes from below the crust, some areas house forests of eerily glowing fungus, whilst others have fresh water lakes and rivers that stretch for miles and wind their way through the tunnels and caverns. What all of these areas have in common is that no light from the sun has ever been cast upon them, and that for the most part they are pitch black.