From Old Norse 'rif' (rib)—reefs are the 'ribs' of the sea floor, ridges running beneath the surface.
A ridge of rock, coral, or sand lying at or near the surface of the water.
From Dutch 'rif' or earlier Old Norse 'rif' (a rib, a reef), literally 'a rib of the sea floor.' The metaphor of underwater rock formations as ribs of the earth is shared across North Germanic languages. Key roots: *rebją (Proto-Germanic: "rib, ridge").