Subterranea is the vast network of underground caverns and tunnels underneath the surface of the world. It is home to a vast range of strange beings and civilizations. Its environment varies vastly from the surface world, making adventuring through it a dangerous expedition. Subterranea is mainly without any source of light save for the occasional luminous rocks or fungi. Large portions of it are also without food or water, making well-stocked food supplies a must-have when exploring uncharted terrain underground.
Just like deep dungeons, the air can grow stale and even poisonous in Subterranea. It could even contain explosive gases from underground volcanic activity or other deadly gases bubbling out of deep fissures in the rock.
Adventurers traversing Subterranea's passages need to be well prepared. Lack of light combined with a tomb-like silence makes traveling something only for the strong-hearted. The nerve-wracking silence makes even experienced adventurers grow agitated after a few days.
Subterranea is extremely dangerous, especially to those not native to it. There are the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost. Most Subterranean inhabitants have highly developed senses other than sight. Food can be extremely difficult to find, and much of the natural vegetation is poisonous.
In addition to those dangers, Subterranea is also the home of many predators and often-hostile races.