Organisms that thrive in what, for most terrestrial life-forms, are intolerably hostile environments.1 The majority of known extremophiles are varieties of archaea and bacteria. They are classified, according to the conditions in which they exist, as thermophiles, hyperthermophiles, psychrophiles, halophiles, acidophiles, alkaliphiles, barophiles, and endoliths. These categories are not mutually exclusive, so that, for example, some endoliths are also thermophiles.