Underdark

The Underdark is an underground system of caves. One entrance is found in the Dreadlands and can be found under the sea between Venumbra's layer and The Blue Veil.
Most of the Octave recommend using a water breathing potion to dive down while Ezra suggests using a giant shark to quickly descend to the ruins that mark the entrance to the Underdark. Be careful of fish-men.