The Lithosphere:see here for map
Controlled by the Monsapple Trading Company, the Lithosphere is a set of eight major inhabited cavern systems:

• Bauluk: annelidan tribes and Monsapple mines
• Carlith: hul'tessaq nation and sandworm farms - rented to the Spicers
• Furrh-Hamand: the bismuth dwarven nation and mines
• Karmand: the alabaster dwarven nation and mines
• Lower Akazad: the main dwarven nation and mines
• Nonton: blindfish nation and waterways
• Zenoth: vermetic tribes and factories

The Grottos: see here for map
Controlled by Hydrargyrum, the Grottos are a series of caves a little further underground holding major city-states. Most are chthonian races, but some are gnomish. There are also some dwarven city-states. All of these have mines, factories, and tech-plants for Hydrargyrum.

The other places in the Grottos that aren't controlled by Hydrargyrum are Quelluria, Nuralle-Ghom, and Calund.

The Hollow: see here for map
Controlled by the Spicers, the Hollow is a vast empty zone underground that has many territories. Of these, the City Under the Stairs is the most important.

The Hypogaeum: see here for map
Controlled by the Littany Corporation, the Hypogaeum is a vast network of caverns that are home to many, many strange beings. Littany uses these as massive mining and gas operations.

Voiddark: see here for map
Controlled by Teloxiq Technologies, the Voiddark is home to beings of void, including the irriton and lesser athaks.

Lower Depths: not owned by anyone

• Barathrum
• Stagnum Ignis
• Inner Core