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.
Today, the Catacombs wind across the underside of the city, touching the Velverdyva in the middle of the Dock District, the Trade District, and near the conjunction of the two. The otyugh has long since been eaten, but the gelatinous cubes prove to be sufficient for the job of waste disposal. The sewer pipes in the streets and homes are both too narrow for the cubes to enter and beyond their reach, while the exits to the river are protected by a series of high steps that the cube cannot climb.
Especially vile criminals are often polymorphed into gelatinous cubes when the population of the Catacombs becomes low. Such people usually lose their own minds in the face of starvation and imprisonment in a sewer. The Academy of Sorcery has one larger access pipe for the purpose of restocking the Catacombs, which is normally kept covered with a heavy stone lid. The Alliance is also rumored to have a similar entrance in one of its secondary headquarters, and the rumors also suggest that they use the tunnel to dispose of bodies.
In any case, the Catacombs are not a safe place to be, and since their narrow drainpipes make them practically useless for infiltrating most buildings, they are left alone (if not taken for granted) by the people of Dyvers.