Reef (Underwater Feature)

A ridge, chain or area of rock, coral or other consolidated material lying at or near the water surface.

Reefs may be biogenic, volcanic, rocky or artificial and can be exposed, drying or fully submerged. Sandbanks and shoals can create similar navigation hazards but are not necessarily reefs because unconsolidated sand alone is not the defining material in all hydrographic usage.

Depth, breaking water, currents and charting quality can vary across a reef, and coral growth or erosion may change it. Mariners should use charted limits, soundings, scale, tidal height and a safe margin rather than assume the visible edge or one stated depth defines the whole hazard.