First appears in: The Stone Flame
Water Processing Seventeen is an infrastructure facility deep within the Under's labyrinth of cement corridors. It is briefly highlighted as the shadowy location where young Rylan first spots a terrifying intruder. He breathlessly reports to Shock Crew that the encounter happened "over by Water Processing Seventeen," describing it as a moment where he saw "some kind of flashing light, then a strange figure in the shadows" that he believed was a monster. Phoenix, Beam, and Dill pass the turn for this facility while tracking the intruder, who is later revealed to be the Remnant warrior, Zel, stumbling through the Under.