Baldur's Gate/Sewers

The sewers of Baldur's Gate were a sanitation system of tunnels beneath the city whose operation was maintained by the Master of Drains and Underways.

Functionality
Due to the city of Baldur's Gate having geographically started life within a steep-sided bowl, the brute majority of its sewer system operated on gravity and pumps.
 * Initial System

This sewer system initially consisted of numerous clay pipes with narrow interiors, which could be as big as in diameter in the largest, lowest sections of the sewers. These pipes were laid in flanged sections that fit into each other and were sealed with concrete. Pipes only got larger in the areas where they met, which were known as junction rooms. Such rooms were typically built in areas where “plugs” were likely to occur in the system, so they had a tendency to back up during storms. Only a few of these rooms had surface shafts with iron rungs set in the walls for use as ladders Those with access shafts always led up to unmortared, rectangular stone "manholes", that were located on the city streets. This ensured that any overflow onto the surface ran down the street, into gratings further away, and back into the system.

The system's pipes ran horizontal, descending in parallel from the plateau around the bowl. They were flushed by cisterns, which were filled by pumps that pulled seawater uphill from the River Chionthar estuary. These pumps were powered by mules trudging in circles.

As the Gate continued to grow over time, the descending length of pipes got increasingly-long and required the installation of "outer pumps," situated further and further away from the bowl. These were necessary to ensure that waste made its way to the edge of the bowl, where gravity could do the rest of the work and dump it out into the River Chionthar.

The outer pumps were traditionally located in the fortified stone outposts of the Watch to ensure their mechanisms and mules were safe from mischief.

The initial system failed to account for the tidal nature of the Gate's river estuary washing untreated sewage back into the intake pumps, creating a cycle of sewage that propagated stench and disease. These issues were solved in part by a filtration system designed by the Church of Gond sometime prior to 1368 DR.
 * Expanded System

This new filtration system placed beneath the Upper City a number of intermediate-flow pumps and stone cellar settling-chambers with pipes and valves spanning the walls, with the chambers divided into three geographical areas (central, eastern, and western). All together these circulated sewage through gratings and settling “wells” to treat it or use it in the flushing of waste. Both the gratings and the bottoms of these wells required regular cleaning.

Those stone cellars notably connected to the Undercellar, the headquarters of the Iron Throne, and Ducal Palace.

The other major addition of their new system was Sewer Keep, a tower complex located in the Seatower district. This building was designed to treat and purify the waste that left the city's sewer system before emptying it out into the River Chionthar.

Sewer Ecology
The creatures most commonly encountered in the central sewers beneath the Upper City in 1368 DR included: gray oozes, green slimes, mustard jellies, ochre jellies, and a tribe of kobolds led by Ratchild.

The creatures most commonly encountered in the western sewers beneath the Upper City in 1368 DR included: ghasts, ghouls, phase spiders, as well as a sentient olive slime named Schlumpsha and his devoted sewerfolk followers.

Other creatures that could be found within the sewers on occasion included gelatinous cubes and rats.

History
Around 1368 DR, an ogre mage took refuge in the sewers and used domesticated carrion crawlers to cause a string of disappearances in eastern Baldur's Gate, luring victims to the sewers and having them eaten by the crawlers after robbing them. Eventually Gorion's Ward was hired by the Flaming Fist to investigate the disappearances and they confronted the ogre mage within the sewers.

Rumors & Legends

 * Rumors abounded of the Blushing Mermaid having a connection to the sewers.
 * The Three Old Kegs was said to contain a secret passage linking it with a sewer shaft that came to the surface near the Black Dragon Gate.