The Sullen Reach is a flooded zone at GLMZ's southern margin, where the 2171 Lake Michigan surge met the pre-existing floodplain of a channelized river system and simply didn't leave. The water is between one and four meters deep across approximately sixty square kilometers of what was once dense urban and industrial fabric. The Reach is not open water — it is a drowned city, its streets still navigable by flat-bottomed boat, its taller buildings still projecting above the waterline, its traffic signals still visible beneath the surface on clear days, still cycling through green and yellow and red on a timer that nobody has turned off and nobody can reach to turn off.

The Sullen Reach is inhabited in the way that very difficult places are inhabited — out of necessity, ingenuity, and the particular human stubbornness that refuses to acknowledge that a place has become uninhabitable. Roughly two thousand people live in structures that project above the waterline: the upper floors of apartment buildings, the rooftops of single-story commercial buildings converted into platforms, purpose-built stilt structures erected on the roofs of submerged buildings. The community is called the Reachfolk by outsiders and just the Reach by themselves. They move by boat, by bridge, by rope line. They fish in their own streets. They dive the submerged lower floors of their own buildings for salvage.

The water is not clean. The Reach is downstream of both the Axiom industrial corridor and the pre-surge Chicago sewage infrastructure, significant portions of which ruptured during the flooding. The specific cocktail of industrial effluent, sewage, and lake water that makes up the Reach has been tested four times by different organizations and produced four different reports, all of which agree on two things: it is not safe for consumption without extensive filtration, and the long-term health effects of regular skin contact are unknown but probably bad. The Reachfolk know this. They have developed filtration systems, skin barrier practices, and a certain fatalism that outsiders mistake for ignorance.