The great city is under the yoke of a drow criminal guild. Matriarch Lady Knuckles saw to it that her underground network was also connected to the Underdark. Six caves in the complex were expanded downwards.
This is a different kind of map. It's the underground system of a metropolis. The sewage system is used for the guild's advantage. They also expanded it with a network of caves interconnected with the sewer hubs and the basements of their safe houses.
This map is important, please do not lose it. It is the only copy and many guards died exploring the underground network to create it.