The matrices and fissures of rocks host abundant microscopic life throughout the Earth’s crust, which is still at an early stage of exploration. While some fissures support simple invertebrates, most organisms are unicellular. Blue-green algae may occur in near-surface layers of rock, but most energy comes from chemical synthesis of minerals. Rates of growth and reproduction are slow, and limited by the supply of energy. At depth, microbes tolerate high pressures and temperatures.