The ancient world
In Hamelin Pool, a part of the Shark Bay World Heritage Area, the water is twice as salty as the open ocean. This hypersaline environment is created by a combination of factors: a shallow sand and seagrass bank, known as the Faure Sill, restricts water flow, while a hot, dry climate leads to high rates of evaporation with very little freshwater input. The salinity here fluctuates but generally ranges from 55 to 70 parts per thousand. These extreme conditions prevent most marine animals from surviving, which allows the slow-growing microbial communities to build structures called stromatolites without being eaten.
The structures themselves look like rounded, rocky domes; they are living biological communities. They are organo-sedimentary structures formed by complex communities of microbes, including cyanobacteria, that trap and bind sediment particles. The microbes also precipitate calcium carbonate from the water, cementing the layers together over time. The process is incredibly slow; radiometric dating shows the stromatolites have a long-term vertical growth rate of about 0.4 millimeters per year. Some of the largest columns, reaching heights of around 350 millimeters, may have taken a thousand years to form.