Networks of predominantly natural tunnels found in nearly all the continents of New Arcadia. They are notoriously treacherous, with magma pools and certain areas compromised by dangerous leakage of The Sea of Storms, the ocean of raw magic keeping New Arcadia aloft, up through cracks in the tunnels. Creatures in the caverns have been found horribly scarred or irrevocably mutated by the wild magic.
The predominant sentient people in the Undercaverns are The Tiefling Clans, seeking shelter from hatred of other races against the descendants of infernal magic users.
To survive in the Undercarverns, people must be quick and cunning, and constantly wary for the environment, predators, and raiders.
Very few entrances from the surface are known, but it rumored that almost every major settlement has a secret passage into the tunnels. The Tiefling Clans guard these secrets jealously, even to the point of death.