Seagrasses, the pale green flowering plants that form meadows on the ocean floor, are home to all manner of life: turtles, fish, squid, seahorses, anemones, crab, dugongs. Yet for vast stretches of the marine world we still don’t know how much there is—and not just around far-flung atolls, but even along the coasts of some of the best-mapped countries on Earth.

Seagrass can stretch for hundreds or thousands of miles, and in areas with clear water that lets sunlight penetrate, may grow far deeper than humans in scuba gear can dive. So when Austin Gallagher, a marine scientist, and his team set out to see just how widespread these meadows were in the Bahamas, they sought help from some of the