Most atolls of the Maldives consist of a large, ring-shaped coral reef. Islands measure only one to two square kilometers in area. Although some of the larger atolls are approximately 50 kilometers long from north to south, and 30 kilometers wide from east to west. The Maldives has no hills, but some islands have dunes which can reach 2.4 meters Islands are too small to have rivers, but small lakes and marshes can be found in some of them.