ARTE/GombessaExpedition2017 At the south end of Fakarava Atoll, a 35-mile-long rectangle of coral in French Polynesia, a narrow channel cuts through the barrier reef. Every June thousands of ...

This story appears in the July 2017 issue of National Geographic magazine ... around Alaska, down to French Polynesia, past the tip of South America, on to Antarctica, and then up north through ...