Stony corals continually lay down a skeleton of calcium carbonate. Like the rings of a tree, this is deposited as a layered structure with the oldest layers deep inside the coral and the newest just under the living tissue on the surface of the coral.

Taking a core of a large coral head allows us to look back into the past without killing the entire colony, and lets us probe climate records and local factors such as sedimentation into the distant past. Very large coral heads can be hundreds of years old.