The Great Way

A never ending infinite tunnel system, that stretches under the whole of Metrus. A railway system connects the cities of Metrus underground, away from the harsh surface air. If one was to enter these tunnels without adequate guides then the likelyhood of a party reaching the other city slims down dramatically. Current maps for this tunnel system are constant under new revision and more tunnels/caverns are being added by the day, meaning that a map you buy from a merchant could be out of date, or even contain misinformation.
The most prominant permenant residents of these caverns are creatures of demonic nature, abberations & mutants are a common sight. Various portals to otherworldy planes and their connection to the underdark make this one of most dangerous places within Ardatia. Of course, occasional a small settlement may have been setup within the caverns. By various outlaws or those no longer allowed in cities, but usually the creatures see to it that these places don't last very long, and those that do are heavily guarded against the terrors of the darkness.