The City in the Lake of Unknown Depths

The cover of the World of Greyhawk folio (1980),
showing Greyhawk and the Lake of Unknown Depths

Greyhawk lore, in various stages of development, is scattered about in various early D&D publications. For example, at least two different products - neither of which is the actual World of Greyhawk product - include references to cities or civilizations under Nyr Dyv, the Lake of Unknown Depths.

The first is the fourth D&D supplement, Eldritch Wizardry, published in 1976. In this booklet, instead of new "ordinary" magic items, Gygax provided us with a slew of Artifacts, each with its own short backstory, some of which include Greyhawk lore. For example, for the Codex of Infinite Planes, we learn:

"Long ago the wizard-cleric who ruled the Isles of Woe lost in the Lake of Unknown Depths used this work to gain knowledge of great power. It is told that this arcane wisdom is what eventually wrought the downfall of the mage-priest and caused the waters to swallow his domain", pg 43 of Eldritch Wizardry (1976).

This was later revised for the 1979 AD&D DMG, with the wizard-cleric becoming "the High Wizard Priest", and the "Lake of Unknown Depths" changed to its formal name, Nyr Dyv, along with a formal reference to the World of Greyhawk product, not yet published.

In the year after Eldritch Wizardry, the game play aid Outdoor Geomorphs: Set One Walled City (1977) was published, and included a short sample key to give DMs an idea of how the set should be used. Allan Grohe has kindly transcribed these for us. One of the city locations is a fish market, the Silvery Mart, and part of the description for it reads:

"If the owner of the fifth booth on the east side (counting up from the north) is engaged in conversation, he will relate a tale about his adventure on the Lake of Unknown Depths, where a very friendly mermaid told him of the City in the Lake. If bribed with no less than 10 gold pieces in goods or cash he will draw out a map showing where he thinks the fabled city lies beneath the waters (he is right), and he will warn whomever it is he is telling about it that there is a dreaded monster guarding the crystal steps down to the city. He can tell nothing more. (See #56, Society of the Sages, for details of which sage knows what about the legend of the City in the Lake)" 

Unfortunately, Area #56 is not included in the brief sample key, so we'll never get to hear the full legend of the City in the Lake directly from the sage, unless we write it ourselves. It's possible Gygax had his earlier story from the Codex of the Infinite Planes in mind, in which case the City in the Lake could be the resting place of the High Wizard Priest of the Isles of Woe. On the other hand, the "crystal steps down to the city" sound like something that was purposefully built, and "fabled city" sounds more wondrous than sinister, so perhaps the two are separate locales under the lake.

This post on the Greyhawkery blog points out just how deep the lake is - over 30,000 feet deep, as deep as the deepest parts of the ocean. Could this depth have been created by the same cataclysm that sank the Isles of Woe?

When the World of Greyhawk was finally published in 1980 (in folio format) and again in 1983 (boxed set), there was no mention of either legend in the description of the Nyr Dyv, with the only fantastic lore being in regard to the the lake monsters:

"The Nyr Dyv is also well known for the monsters which inhabit its waters. Deep beneath the surface lurk huge creatures which prey upon unwary sailors or anyone so unlucky as to fall into the water. Warcraft, and occasionally merchants or bargees will bring back such monsters as trophies, as constant warfare upon these creatures is necessary to make the lake useful and usable" (pg 25)

Illustration by David Sutherland from the 1980 WoG folio,
showing a barge being attacked by a lake monster

2018 Update: One other bit of lore regarding the Nyr Dyv is that S1 Tomb of Horrors suggests "An island (unmapped) in the Nyr Dyv" as one possible location for the Tomb.