National Geographic photographer Joel Sartore is on a mission in what may be the biggest assignment of his life. For decades Sartore has photographed some of the world’s wildest places for National Geographic, from Antarctica to the Amazon rain forest.

But Sartore didn’t feel like he was making a dent in species conservation. Eight years ago, Sartore began a project called Photo Ark, which aims to photograph the roughly 6,000 species currently inhabiting zoos around the world. Now he’s more than halfway there, at 3,050 captive species and counting.

A lot of advance work and behind-the-scenes prep happens in concert with zoo employees to ensure the animals are comfortable before he starts photographing. For each picture he installs a stark