The barrier reef that forms the basis of Aitutaki is roughly the shape of an equilateral triangle with sides 12 kilometres (7.5 mi) in length. The southern edge of the triangle is almost totally below the surface of the ocean, and the eastern side is composed of a string of small islands including Mangere, Akaiami, and Tekopua.

The western side of the atoll contains many of Aitutaki’s important features including a boat passage through the barrier reef allowing for anchorage close to shore at Arutanga. Towards the south of the side is a small break in the barrier reef, allowing access for small boats to the lagoon which covers most of the southern part of the triangle. Further to the north is the bulk of the main island.

source: wikipedia.org