are single-celled algae with a cell wall made of silica (glass), and make up the
largest part of the plankton on which whales
and other aquatic animals feed. When the algae dies, their shells pile
up on the sea bottom. Over time this build up becomes diatomaceous earth or diatomite,
which is used in filters, abrasives, insulation and explosives. Most
limestone and many petroleum products are of diatom origin.