Stromatolites. Coloured scanning electron micrograph (SEM) of the surface of a stromatolite. Stromatolites are mineralised microbial communities, formed from cyanobacteria (formerly known as blue-green algae). They are formed over thousands of years as the cyanobacteria trap detritus and sediment, forming large living rafts known as microbial mats. The cyanobacteria also secrete calcium carbonate, which causes the mats to mineralise, forming rock-like structures. Sample taken from Lake Thetis, Western Australia. Magnification: x1250 when printed at 10 centimetres wide.