Underneath Collossapolos is a vast labyrinth of chambers and passages called The Twelve Tiers. This area also contains the city's sewers which drain into the Underdark.
Most of The Twelve Tiers are unmapped. This is restriction imposed by the Orchish Empire law of Map Restrictions; a security measure in case some hostiles want to use the Underdark to attack the city.
There are twelve subterranean levels under Collossapolos. Most of these labyrinths and passages were expanded by Monty the Mad during his search for a powerful relic called Tiamats Spur.
The twelve levels under Collossapolos:
|1||The Rat Catacombs|
|2||Ancient Shrine Of The Toad|
|3||The Fungi Maze|
|4||Hold Of The Dark Orbs|
|6||The Gas Vaults|
|7||The Boiling Lake|
|8||The Deep Ones|
|9||The Long Walks|