Stromatolites. These mineralised microbial communities are formed from blue-green algae, or cyanobacteria. Over the last 4000 years, algae in this pool have trapped detritus and sediment, forming large, living rafts known as microbial mats. The secretion of calcium carbonate by the algae caused the mats to mineralise, forming the structures seen here. These stromatolites are known as living fossils, because the process of mat formation and mineralisation continues today. Photographed in Hamelin Pool, Shark Bay, Western Australia.