A type of Free Zone found in some regions outside the jurisdiction of the civilized galaxy. Sewerslums are where the rejects gather and mutate.
Victims of bionanite mutagens. Especially unfortunate firstvers. Toxic wastes and dangerous hormones get dumped there, where it's cheaper than recycling them economically. Criminals end up there, and other less wholesome beings created by rogue hobbyists. Within that place they mutate and warp, usually dying but sometimes giving birth to other things, things that don't bear thinking about.
A sewerslum can be a shanty town adjacent a local waste recycle dump; a ghetto slum in an otherwise respectable city; proper habitat with running water, power, recycled air, and full nanofaculties; a simple hollowed out asteroid; a hastily erected habitat dome; an enclave hidden in a terraseeded jungle; a cave cut out of the rock; or anything else. But no one even pretends to have any idea what's in a sewer slum, and no one really wants to find out. Those that do are either very dedicated scientists or those who have unpleasant business in mind. Sometimes nasty creatures will come from the sewerslums out into the relatively normal world and wreck havoc. For this reason all sewerslums are patrolled on the borders by guards or (in the case of space sewerslums) by armed drones. In a few rare cases sewerslums have grown to encompass whole planets and solar systems with the ordinary people fleeing.
Note: "Sewerslum" is actually a derogatory term used by outsiders (the straights or normals) who don't understand the "zone", the "territory")