In the lagoon, numerous coral heads present hazards to navigation. The shallow reef shelf surrounding the island on the ocean side offers no ocean-side anchorage. The channel and anchorage areas in the northern half of the lagoon are dredged, along with the pre-1971 ship turning basin. Significant saltwater wetlands called barachois exist in the southern half of the lagoon. These small lagoons off of the main lagoon are filled with seawater at high tide and dry at low tide. Scientific expeditions in 1996 and 2006 described the lagoon and surrounding waters of Diego Garcia, along with the rest of the Chagos Archipelago, as "exceptionally unpolluted" and "pristine".