Geologists estimate that Mangaia is at least 18 million years old, making it the oldest island in the Pacific. It's the second largest of the Cook Islands and the furthest south of the Group. It rises 15,600 feet (4750 m) above the ocean floor. It has a central volcanic plateau and, like many of the southern islands in the Cooks, it is surrounded by a high ring of cliffs of fossilised coral, called the makatea, in this case 200 feet (60 m) high. It looks beautiful in the picture on the right, but the coral is razor sharp.