These air-filled voids beneath the ground are simple low-productivity ecosystems limited by an absence of solar energy, except around their openings to the surface. Food chains consequently lack plants and herbivores. Microbes in biofilms are the dominant life forms, but some caves have invertebrate detritivore and predators, or temporary vertebrate inhabitants. Their limited energy comes from organic material imported by seepage or animal movements, and bacteria that synthesise chemical energy from rocks. They are found on all major land masses, most commonly in carbonate rocks or lava tubes.