>>96773175
I was thinking about the sewers in splicers. In the campaign I had running I had required land nav rolls because it was a maze that led everywhere with lots of flooded chambers and cisterns. Eventually I had a old guy with agoroohobia act as a guide NPC for the party since we figured that navigating by stars or sense of direction does not work in underground tubes.

For that extent I also though of a slug warming designed to travel through the tubes quickly, feeding off the crude in the pipes.