This coral reef resurrected itself — and showed scientists how to replicate it

With measuring tape and notepad, marine ecologist Enric Ballesteros surveys the organisms living on a healthy reef in the islands. When author Enric Sala and his team first visited here in 2009, they found these reefs in a pristine state, with a profusion of species, many of them rare. (Jon Betz/National Geographic)

Though they may not know it, about half a billion people worldwide depend on the ecosystems created and sustained by corals. And with climate change threatening coral's survival, marine scientist Enric Sala had a goal that might have seemed impossible.

"We wanted to get into a time machine, go back hundreds of years and actually see a coral reef like they used to be everywhere, before we started exploiting them and polluting them and killing them all over the world," Sala said.