Beneath the surface of Mythosa, there are endless miles of tunnels, cavern complexes, and myriad underground hollows. Some are close to the surface or inside of mountains, while others are so deep that they are as alien to the sunlit world as would be another plane. The entirety of the subterranean world is known as the "Underworld".
The Underworld is home to a great variety of plant, fungal, and animal life, much of which is intelligent. Orcs, giants, dragons, and the Drakari are just a few of the better known races that inhabit the world beneath the ground. It also contains stunning geological wonders and vast quantities of metals and gems.
Naturally, the Underworld is not without its dangers; in many cases, journeying into the Underworld is more dangerous than venturing into the wilderness of Karnathas or Zarkhir. Besides the threat posed by various monsters and plants, there are also dangers from cave-ins, poisonous gases, suffocation, and other things only possible when traversing a cave or passage beneath thousands of feet of rock.