Reef

A reef is a rock, sandbar, or other feature below the water surface (less than 80 meters below low water).

A typical coral reef and what lives there

The best-known reefs are the coral reefs in tropical waters. They are built by corals and calcareous algae. Artificial reefs such as shipwrecks are sometimes created to attract fish and other organisms.

Reefs in the Louisiade Archipelago