The Catacombs of Dyvers
The bedrock to the southwest of the Nyr Dyv is primarily limestone. This relatively soft rock has made it easy for the city to establish an effective sewer system. Originally, waste and refuse were dumped into the river, but shortly after Furyondy declared its independence from the Great Kingdom, the mayor of Dyvers (a practical and somewhat foppish wizard named Karmas) set into action a plan to expand upon some of the limestone caves that dotted the shore of the Velverdyva.
After two years of magical and mundane labor, the Catacombs were completed. Far less ominous than their name implies, the Catacombs were a series of linked tunnels under the city connected to small drains in some of the larger streets. The tunnels also connected to the riverfront, allowing some limited circulation of materials. Captured monsters (mainly gelatinous cubes and an otyugh) were then introduced to the Catacombs in order to process the unsavory materials that were thereafter dumped into them via the narrow street drains. As the city grew, more tunnels were completed, and some of the larger buildings (such as the Old Palace and the Royal Furyondian Barracks) were given their own sewage drains.
Sean K. Reynolds. Greyhawk Adventures, Slavers, 2000