The Atlantic Ocean holds a secret: a patch of calm water ringed by swift currents, sitting about 590 miles east of Florida yet never touching land. Known as the Sargasso Sea, sailors have crossed it for centuries, but few notice the border when they slip into glassy indigo waters.

Those who linger find the surface scattered with golden-brown seaweed – Sargassum – named for the Portuguese word sargaço, a type of grape-like algae. The plants bob in slow motion, rolling gently like tumbleweeds on a prairie of water.