From a distance, Lake Hillier of Australia’s Recherche Archipelago looks like a swath of solid bubble-gum pink. Draw closer, and the color takes on a more watery, translucent quality, but remains unmistakably pink.

Whereas the causes behind the unusual coloring of other pink lakes, such as the nearby Pink Lake and Senegal’s Lake Retba, have been definitively confirmed, the reason for Lake Hillier’s color remains a mystery. Theories abound, of course.

Some speculate that Lake Hillier’s color, like that of the other lakes, is the result of high salinity combined with the presence of a salt-loving algae species known as Dunaliella salina and pink bacteria known as halobacteria. Unlike other pink lakes, however, which regularly change colors in accordance with temperature fluctuations, Lake Hillier maintains its pink shade year-round. The water even retains its rosy hue when bottled.

Whatever the cause, the water does not appear to pose any danger to humans. Swimming in it is not possible anyway, since the island is used only for research purposes and tourists can admire it only from above on helicopter rides.