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A stromatolite collected from the 3,600- to 3,200-million-year-old Paleoarchean Strelley Pool Chert of Western Australia, Australia. Stromatolites are formed over the years by mats (1–10 mm in thickness) of microorganisms (cyanobacteria among others) found in shallow, mainly marine waters. The microorganisms precipitate mineral particles, which makes the mat thicken, but only the upper part survives. Most stromatolites display characteristically layered structures. Only the layers are visible to the naked eye. Photo credit: Didier Descouens