A reef is a ridge of rock, coral, or sand that lies just below or at the surface of the ocean. Coral reefs are built over thousands of years by tiny animals called coral polyps, which create hard skeletons that stack up into vast underwater structures. The Great Barrier Reef off Australia's coast stretches over 1,400 miles.

Reefs are among the most diverse ecosystems on Earth, home to thousands of species of fish, sea turtles, sharks, and other marine life. They protect coastlines from storms by breaking up powerful waves before they reach shore. Sailors have long needed to navigate carefully around reefs, since a ship striking a reef can tear open its hull and sink.

The word also appears in sailing terminology: to reef a sail means to reduce its size by rolling or folding part of it, usually when the wind gets too strong. A crew might reef the mainsail during a storm to prevent damage.