The dominant reef-building organisms of previous times were corals. Algae cells lived symbiotically with the coral, supplying oxygen and carbohydrates used in the production of calcium. With the recent glacial period, in 5 million AD, came massive climatic disruption. The seas filled with mud, depriving the algae of the sunlight they needed for survival. Without the algae and the essential nutrients they provided, the corals also became extinct some time later. Now, tens of millions of years later, there are large areas of warm, shallow water and conditions are right for reefs to develop once more. This time, the reefs are formed not from coral, but from a prolific species of red algae.