Karst environments can be grouped into three broad categories, based on their vertical position in the landscape. There are surface habitats, ones exposed to light; there are shallow subterranean (aphotic) habitats oft en with small to intermediate sized spaces; there are deep subterranean habitats (caves) with large sized spaces. Faunal records are most complete for caves, and on a global basis, more than 10,000 species are limited to this habitat. Hundreds of other species, especially bats, depend on caves for some part of their life cycle. A large, but most unknown number of species are limited to shallow subterranean habitats in karst, such as epikarst and the milieu souterrain superficiel. Species in both these categories of habitats typically show a number of morphological adaptations for life in darkness, including loss of eyes and pigment, and elaboration of extra-optic sensory structures. Surface habitats, such as sinkholes, karst springs, thin soils, and rock faces, are habitats, but not always recognized as karst habitats. Both aphotic karst habitats and twilight habitats (such as open air pits) may serve as important temporary refuges for organisms avoiding temperature extremes on the surface.