The Undertow is an Old Harbor gang of about 60 members that controls the waterfront's most dangerous stretch — the derelict section of shoreline known as the Drowning Mile, where abandoned industrial piers, collapsed seawalls, and flooded basements create a landscape that is half land and half water and entirely lawless. The gang takes its name from the treacherous currents that make the Drowning Mile's waters lethal to the unprepared, and from their preferred method of disposing of enemies.

The Undertow's primary business is waterborne smuggling — using small boats and submersible drones to move contraband along the coastline, bypassing the port where the Harbor Rats charge tolls. They also control a modest extortion operation targeting the Drowning Mile's desperate inhabitants (people living in partially flooded structures who have nowhere else to go), run an illegal fishing operation that provides food to Old Harbor's poorest residents, and offer a disposal service that the Bleach Boys respect for its simplicity: the ocean asks no questions.

The gang is hardened by its environment. Undertow members live in the Drowning Mile's worst conditions — wet, cold, exposed to industrial toxins — and this shared hardship creates a bond that more comfortable criminal organizations lack. They are fewer and poorer than many Shelf gangs, but they fight with a ferocity born from having nothing to lose and nowhere to retreat to.