A lagoon is a shallow body of water separated from a larger, open body of water. It is typically associated with the ocean, such as coastal lagoons and coral reef lagoons. Lagoon also can be used to describe shallow areas of liquid waste material as in sewage lagoons. Oceanic lagoons can be formed in several ways. Coastal lagoons are typically found along coastlines where there are sand bars or barrier islands that separate the open ocean from the near shore body of water. Coral reef lagoons can form in two ways. The first type is found in barrier reefs such as those in Australia and Belize, where there is a body of water (lagoon) which is separated from the open ocean by the reef formed many miles off shore. Another type of lagoon is that formed in the center of atolls, which are circular or horse-shoe shaped bodies of water in the middle of partially sunken volcanic islands with coral reefs growing around their periphery. Some of these atoll lagoons are more than 30 mi (50 km) across and have breathtaking visibility, thus providing superb sites for SCUBA diving.