Florida Panther (Puma concolor coryi), Lowry Park Zoo, Tampa, Florida. [Photo by Joel Sartore/National Geographic Photo Ark]

Photo Ark is Joel Sartore's decade-long project to capture intimate portraits of the estimated 12,000 species in the world's zoos and aquariums – with an emphasis on those facing extinction.

To date, he has visited 40 countries in his quest to create this photo archive of global biodiversity. Early this month, he just completed the intimate portrait of the 6,000th species, a proboscis monkey at a zoo in Singapore.

The compelling eye contact of the animals, stripped of their habitats, has proved so popular that Sartore's images have been projected on to the Empire State building and St Peter's basilica.