This great lake, also known as the Lake of Unknown Depths, is a strategic body of water. Across it, much trade is plied between Greyhawk, Dyvers, Furyondy, Urnst, and the lands beyond as fas as Perrenland and the Theocracy. As the map shows, many major ports and navigable inlets and outlets throng the waters of the Nyr Dyv. The navies of Furyondy and Urnst still dominate this great lake, protecting the shipping that travels it; Iuz has not yet sought to extend any naval strength to the Nyr Dyv.

One of the most unusual features of the Nyr Dyv is its people. These folk make their homes on great barges anchored along the shores. They conduct much of the day-to-day conveyance of trade along them. These are the Rhennee, gypsy waterfolk who are mostly thieves and occasionally pirates.

Each Rhennee barge is crewed by a "lord" who shares his cabin with his family, and "cousins" (who may or may not be related by blood) who find living space on or below deck. A typical barge contains some 15-20 souls. The Rhennee have both a special cant of their own and a subtle communication systems using flags and lanters.

The Nyr Dyv is also home to many monsters. Freshwater serpents and octopi have often been reported, as have freshwater aquatic umber hulks and worse. A notable feature of a more pleasant aspect is a large community of selkie off the coast of Oldred, who do their best to warn sailors of possible hazards of this type. The selkie are known to be unfriendly towards Rhennee.