Plural of reef; underwater ridges of rock, coral, or sand that rise near the ocean surface.
From Middle Dutch 'rif' or Old Norse 'rif', both meaning rib or ridge. The nautical sense developed because reefs appear as ridges or ribs beneath the water surface.
Coral reefs are among Earth's oldest ecosystems, with some formations over 400 million years old. The Great Barrier Reef is so large it can be seen from space and is technically the world's largest living structure.
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