Lake Hillier, is a lake on Middle Island, the largest of the islands and islets that make up the Recherche Archipelago, Western Australia.
The most notable feature of this lake is its flamboyant colour, rose pink. The color is permanent, as it does not alter when the water is taken in a container. The length of the lake is about six hundred meters. A narrow strip of land composed of sand dunes covered by vegetation separates it from the ocean. The lake is surrounded by a rim of white salt and a dense woodland of paperbark and eucalypt trees with sand dunes separating the lake from the Southern Ocean to the north.
The island and lake are thought to have been first charted by the Flinders expedition in 1802. Captain Flinders is said to have observed the pink lake after ascending the island's peak. John Thistle, the ships master, collected some of the lake's water which he found to be saturated with salt. The reason for the lake's colour is still under investigation, and so far no one has come up with a reasonable explanation. However, the most probable explanation according to some scientists involves the low nutrient concentrations and growth of Dunaliella salina & halobacterium. The lake is one of the natural wonders of Australia..