Stromatolites are literally layered rock. The rock consists of layers of calcareous material which are formed by the prolific growth of micro-organisms such as cyanobacteria (formerly known as blue-green algae), which are invisible to the human eye. These single-celled organisms are the earliest known forms of life on earth with a lineage dating back 3,500 million years. There are over 50 species of cyanobacteria found at Hamelin Pool , at Shark Bay in Western Australia.
The bacteria, which builds the stromatolites, rely on the energy provided by sunlight to power their feeding and growth. At night, when the process ceases, the silt brought in from the tide, settles on the surface of the bacterial colony, trapping sediment by a sticky film of mucus which each cell secretes. At dawn, when the sun rises, the bacteria (which are hair-like) push their way through the silt grains and re-establish the colony on the top surface again. 
The calcium waste that they secrete is left behind in the processes and hardens into a cement. Building up layer by layer this forms the basis for the mounds which eventually form the column structures. The growth rate of these organisms is believed to be approximately half a millimetre per year, making many of the structures several thousands of years old. However, as the organisms live only on the upper surfaces, the height of stromatolites close to the shoreline are limited by the tide, as they require regular immersion in the water for survival. There are four main factors required for the survival of stromatolites, the site must be warm, sheltered, highly mineralised and have few predators. Hamelin Pool is the perfect location because it covers all of the survival requirements, extreme salinity of the seawater, limited circulation of water and the presence of calcium carbonate which, as a result, provides an environment in which predators cannot survive.
Australia’s second largest population of stromatolites or more correctly thrombolites can be found at Lake Clifton (near Mandurah) which are smaller and younger.
The presence of stromatolites was a major factor in Shark Bay being declared a World Heritage Site in 1991. It is one of only two places in the world where living marine stromatolites occur, the other is in the Lee Stocking Island, Bahamas. Wooden boardwalks have been constructed in the Hamelin Pool area to protect the delicate structures from being damaged by the many tourists who visit.