Every year hundreds of thousands of people gather on Jindo Island at the southern tip of the Korean Peninsula to watch the sea part, revealing a 1.8-mile causeway that permits them to walk to the nearby island of Modo, where they dig for clams.

Legend tells that Yongwang, the ocean god, split the sea to permit an old woman to rejoin her family. But National Geographic explains that the truth lies in tidal harmonics.