From 20,000 years ago until 6,000 years the sea level began to rise. As it rose, the corals could grow higher on the hills of the coastal plain. 13,000 years ago the sea level was 60 metres lower than the present day. Corals began to grow around the hills of the coastal plain. These became continental islands. As the sea level rose higher, most of these continental islands were covered by water. The corals could then grow over the hills. This formed the present cays and reefs. Sea level on the Great Barrier Reef has not risen much in the last 6,000 years. The CRC Reef Research Centre has worked out that the present, living reef is 6,000 to 8,000 years old.