Moondeep Sea

The Moondeep Sea was a subterranean lake approximately 240 miles (386 km) wide found in the Underdark beneath the Galena Mountains.

Geography
The cavern which contained it was known as the Vault of Gnashing Teeth, so named for the countless animated skulls stuck to the cavern's ceiling.

The Moondeep Sea was known for its giant aquatic spiders and the city of V'elddrinnsshar‎, which lay on a mile-wide island dominated by a plateau in the middle of the lake.