Every surfer knows the reef walk. One eye peeled on the lineup, and the waves (hopefully) crunching, roping or sliding down the reef. The other eye downcast, picking a line between urchins, toe-stubbing boulders, and ankle-wrenching potholes. Many surfers consider that just dead ground; an obstacle between you and the ocean and a surf.

Except it turns out something else does that exact walk, on purpose, for a living. Somewhere off the southeastern coast of Papua New Guinea, in the kind of skinny, knee-deep reef water you’d normally just stumble straight over, there’s a shark doing push-ups across the seafloor. Not swimming. Walking. On its fins.…