Stromatolites in a lagoon. These special rock-like structures are Earths oldest fossils and can often be found in shallow water. They are formed by bacteria such as cyanobacteria, a single-celled photosynthesising microbe. The microorganisms produce adhesive compounds that cement sand and other rocky materials to form mineral microbial mats. These mats build up layer by layer, growing gradually over time to form stromatolites. Photographed in the the Tebenquiche Lagoon in Salar de Atacama, San Pedro de Atacama, Atacama Desert, Chile.