Recent surveys reveal that Australia’s Great Barrier Reef has suffered the second most severe coral bleaching ever recorded there, The Washington Post reports. The corals turned white because ocean waters warmed beyond their tolerance range, reaching the highest levels ever observed in February, scientists say. In reaction to heat stress, coral polyps eject algae that normally live inside their tissues and help them survive. The resulting bleaching kills many corals and hampers growth and reproduction of survivors. It’s the third “mass bleaching event” to hit the reef in just 5 years; the overall severity of bleaching in 2016 was worse, but the damage done in recent summer months covered a wider area, including relatively cooler areas in the reef’s southern region. Observers conducted aerial surveys of 1036 reef locations in March. They found 25% were severely affected, with more than 60% of each reef’s corals bleached; an additional one-third of locations had more modest levels of bleaching. The scientists will next monitor how many of the bleached corals die this year. In 2016, more than half of the severely bleached coral in the reef’s northern section died.