An underwater barrier is a physical structure that is built to prevent the movement of water or other substances from one area to another. A SANDBANK is an underwater ridge or mound of sand, often found in shallow waters. It is usually formed by the action of waves and currents. A REEF is an underwater formation of rocks, coral, or sand that can form a barrier to ships or other vessels. All three of these structures can be used as an underwater barrier to protect coastlines from erosion or to prevent the spread of pollutants.