The cell walls of diatoms are composed of inorganic silica, so they do not decompose. When diatom cells die, their silica cells sink to the bottom of the wetland or lake or ocean. Because the cell walls are inorganic, they can be preserved over long periods of time - up to tens of millions of years. Unlike many soft bodied organisms, the cell walls are not replaced by another mineral. Diatom fossils are preserved as their original silica cell wall. Other organisms with mineralized structures include bryozoans, which are preserved as their original calcite skeletons in the fossil record. Radiolarians are another type of organism that possess silica stuctures, within their cells. All of these remains of organisms can persist, given the right conditions, and appear as fossil. Paleolimnologists and geologists examine fossil diatoms along with other fossil organisims, and they help us to understand past conditions on earth.