Saltgrave Sublevel occupies the drowned basement infrastructure of what was once Chicago's South Loop transit interchange — a labyrinth of flooded pedestrian tunnels and former parking structures sealed off after the 2147 Lake Surge and never properly reclaimed. Brine from Lake Michigan seeps through cracked concrete walls, leaving thick white salt deposits that encrust every surface like mineral frost. Salvagers broke through from below in the 2170s and have since carved out a thriving gray-market settlement across four interconnected levels, using repurposed buoyancy platforms to keep living quarters above the standing water.

The settlement is famous — or infamous — for its salt trade. Saltgrave's residents harvest the mineral deposits and process them for industrial use, food preservation, and certain chemical synthesis operations that corponations prefer not to conduct in their own facilities. Zheng-Dao Bioelectric maintains a quiet purchasing agreement with Saltgrave's council, though neither party acknowledges it publicly. The deeper levels, perpetually flooded and lit only by bioluminescent algae cultivated along the walls, serve as neutral meeting ground for parties who cannot afford to be seen in any official venue.