Tides & currents moving the bottom (sand, etc.) around, mostly. But it's also possible for a coral reef to have grown taller. Or a volcano, I suppose. :) NOAA'

Tides & currents moving the bottom (sand, etc.) around, mostly. But it's also possible for a coral reef to have grown taller. Or a volcano, I suppose. :) NOAA's Chart No. 1 explains the symbols used on charts. https://www.nauticalcharts.noaa.gov/publications/docs/us-chart-1/ChartNo1.pdf https://www.nauticalcharts.noaa.gov/publications/docs/us-cha...