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