Coral reefs comprise just 1 percent of the ocean floor yet they are home to 25 percent of the world’s marine fish, a growing source of protein for people. But reefs are imperiled by a range of threats including warming waters, acidifying seas, destructive fishing methods, and agricultural and other runoff.

Moreover, scientists have only a rough idea of the extent of reefs worldwide; a reef thought to be 1,000 acres might be 1,500 or just 500. Of the reefs that have been accurately mapped, little is known about their health, the kinds of fish that live there, or the composition of coral species.

The problem is seawater. The oceans are vast, making reefs hard to pinpoint, and the water’s surface is difficult for satellite and airborne cameras to see through.