Diatoms are microscopic algae of the phylum Chrysophyta in the kingdom Protista. Almost all diatoms are single-celled algae and dwell in both fresh- and saltwater; they are abundant in the cold waters of the northern Pacific and the Antarctic Oceans. Diatoms are yellow or brown in color and are an important food source for marine plankton and many small animals. Diatoms have hard cell walls; these “shells” are made from silica that has been extracted from the water. It is unclear how the extraction of silica from water is accomplished. When they die, their glassy shells—called frustules—sink to the bottom of the sea and harden into rock called diatomite. One of the most famous and accessible diatomites is the Monterey Formation along the coast of central and southern California. Diatoms are familiar to gardeners, too—since diatomaceous earth is often used to control garden insect pests.