Darklake

South and east of Mantol-Derith, Darklake is a series of connected bodies of water that were formerly linked by waterfalls but are now governed by locks created by dwarven engineers. The caverns are used as a relatively safe passage by duergar, although the place has its share of monsters (mainly freshwater trolls and a type of evil intelligent freshwater manta ray). There may be a portal that connects the Darklake to one of the underground rivers servicing Skullport, and if the portal and its activation key can be discovered, it would greatly facilitate trade between the Underdark and the surface.