The 3rd Global Coral Bleaching Event was the longest and most damaging coral die-off in recorded history. It’s also the most widespread: from late 2014 to mid 2017, excessively high ocean temperatures devastated corals in all the major oceans. Many healthy reefs are now graveyards. Yet some corals have proved resilient to the deadly bleaching. By working to understand how (and why) corals bleach, we can better protect coral reefs around the world, and may find the key to saving them for the future.