In the Underdark region Drarthiel is a vast body of water tainted from exposure to the Abyssal world Arioch. This comes from abyssal energies bleeding across a rift. In the Demon Spawn War, demons opened an inter-system rift that came to be known as the Arioch Cloud. On Bal-Kriav's side, under what was then called Curatoch, the rift's opening was susceptible to flooding. Demon and aboleth engineers under Athroond built canals and pumps to move this water from the mining pit to the nearby sea. Even after the Arioch Cloud was frozen in time, and the passing of thousands of years, the waters of this Underdark sea remain tainted from the Abyssal waters that once leaked freely from one world to another. The Green Nebulous is also plagued with plants native to the world Arioch. They adapted to the subterranean conditions becoming a nuisance to travel across the sea. Some are predatory plants that even of animal intelligence have an evilness about them, killing for the sheer joy of it.
Irdtraxians frequently travel this sea on massive barges. They journey to and from distant shores, plying their mining skills on ore veins, raiding deep gnome and drow settlements, and gathering sustenance for their evil empire.
The Great Tradeway passes over the lake through upper subterranean passages - the Houndaer Overpass. There are gaping holes in this passage that descend all the way to the dark sea below. The drop ranges from several hundred feet to a mile at the highest point. Going north, the Great Tradeway descends to a very wide walkway that is at some points almost level with the waters of the sea. As a result of kuo-toan attacks from this area, Undercity and Dhaunril'yraen built the Tanaralda wall. This wall protects merchants and travelers on this part of the Great Tradeway from hostilities coming from the Green Nebulous.