Darklake

South and east of Mantol-Derith, Darklake was a series of connected bodies of water that were formerly linked by waterfalls but later governed by locks created by dwarven engineers.

Inhabitants
The caverns were used as a relatively safe passage by duergar, although the place had its share of monsters (mainly freshwater trolls and a type of evil intelligent freshwater manta ray). There may have been a portal connecting the Darklake to one of the underground rivers servicing Skullport, which if discovered could greatly facilitate trade between the Underdark and the surface.