Stromatolites in the Tebenquiche Lagoon in San Pedro de Atacama, Atacama Desert, Chile. 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.