The Hinge Gate

Every morning, Wen replaced one flexible rib on the inspection robot and tested its turn in a bucket. The machine had no propeller. Its paired fins pushed it through flooded passages, and its segmented jaw could open around a cable without biting through it. The design came from a fish that had lived before the first forests.

A near-complete fossil had given the engineers enough anatomy to try a new body plan, though nobody could agree exactly where the animal belonged on the family tree. The robot's first trial had failed in a straight pipe; it worked only when the water forced it to turn. Wen had spent months tuning the fins, while her daughter measured each pass from the other side of the flood gate.

Today the gate was stuck halfway open. The city's nursery pumps depended on it, and the tide would reverse in twenty minutes. Wen could send the robot into the narrow hinge gap, where the old design could not bend, or wait for divers and lose the nursery's water for a day. She clipped the last rib into place. The robot folded its fins and slipped behind the gate. On Wen's screen, its camera showed a second passage beyond the hinge.