A before-and-after image shows the damage done by a coral bleaching event. (NETFLIX)

Coral bleaching turns ­once-colorful reefs into eerie wastelands. A new documentary shows the stark truth behind bleaching, taking you underwater to witness what warming waters do to corals.

“Chasing Coral” is director Jeff Orlowski’s follow-up to his 2012 film “Chasing Ice,” which documented melting glaciers in sobering time-lapse. This time, Orlow­ski and an international team headed into the ocean for views of pale, abandoned reefs that once teemed with life.

The culprit: coral bleaching, which occurs when warm water puts stress on corals. Rising temperatures cause the corals to expel the algae that provide their vivid colors. As a result, they become ghosts of their former selves, susceptible to diseases, reproductive problems and death.

Climate change drives coral bleaching, and in 2015 the world experienced its third documented bleaching event, which wiped out huge swaths of corals. Orlowski gathered more than 650 hours of underwater footage of vanishing coral for the film.

The result is half elegy, half call to action. And although news of the likely end of the coral bleaching event is promising, the film is a reminder that warming waters pose a serious threat to the corals that survived that turmoil.

Coral reefs’ otherworldly ecosystems are easily overlooked because you need to go underwater to see them. With its arresting views of healthy corals and their lifeless counterparts, “Chasing Coral” makes it impossible to look away.

The film debuts in New York and Los Angeles on Friday, when it will also premiere on Netflix.