The coral may not live up to more than 30m deep. However, there are reefs on plinths much more profound. The first colonies of polyps living forms are indeed close to the coast, but while the sea level is rising or that the ocean floor is sinking, new settlements are growing on the skeletons of dead people.

The Great Barrier Reef is relatively young. Scientists believe that 18000 years ago, during the ice age, the water level has dropped, leaving an emergence of hills. The coral then took shape on the continental shelf, in front of these hills.

At the end of the ice age, 9000 years ago, the ocean rose and flooded the hills again. The coral colonies were able to colonize new areas, including on top of ancient hills. At the same time, the oldest settlements followed the elevation of the water, leading new corals to develop over the dead coral.

Since then, over the centuries, the "walls" of corals were formed, amounting reefs. Of the ancient hills appeared platures submerged coral, fringing reefs and cays.

- Coral Platures
They are formed in shallow water and grow horizontally rather than vertically, taking circular or elliptical forms.
- Cayes
The keys are islands of sand composed of limestone debris, including shells and skeletons of polyps, that the waves reduced to powder
- Fringing reefs
They are formed near the coast, surrounding the islands or along the littorals.