The silica cell walls of diatoms are inorganic, so they do not decompose. When diatoms die, they sink to the bottom of the wetland or lake or ocean. The glass cell walls can be preserved over long periods of time, up to tens of millions of years. Diatoms are not true fossils, because the cell walls are not fossilized, or replaced by another mineral. The silica cell walls accumulate in the bottom of lakes and oceans. Paleolimnologists and geologists examine diatom species in lakes and ocean sediments, and the species are used to interpret past conditions.