Jeff Orlowski's suspenseful and heartbreaking documentary focuses on the phenomenon of "coral bleaching"-the process by which dying corals turn white and fall apart.

Which makes it all the more surprising that the documentary ends on the optimistic note that it does.

A story of passionate pioneers who love reefs becomes a clarion call for change and one of the year's most essential movies.

Through time-lapse and stunning underwater photography, we get an eye-opening look at what’s happening to our ocean’s backbone without descending into a politicized debate about climate change.