If you haven’t seen a coral reef for yourself, it can be hard to tell what the fuss over the global coral crisis is all about. From above, a healthy reef is dark, a shadow in the water, soaking up the sun against the pale sand in shallow, clear water. When a reef starts to bleach, it turns visibly white — often the crown of the reef will go first, where the coral in the shallowest water experience the hottest temperatures, leaving white speckles ringed in the darker, deeper corals. From above, these bleached reefs look like long streamers of white rubble against the blue backdrop.

These days, bleached reefs are what you’re most likely to see. Terry Hughes, a coral researcher at James Cook University, convened a task force that carried out an aerial study to figure out just how many reefs are still healthy in the northern section of the 1,400-mile-long Great Barrier Reef. Hughes and company found that out of 520 reefs surveyed, 95 percent were experiencing “severe” bleaching. In fact, Hughes could identify only four reefs that showed no signs of bleaching.

Allow me to do some easy, but alarming math: That means less than 1 percent of the reefs surveyed in the world’s most famous coral reef are as healthy as they should be.