Thought to be among the oldest living things on Earth, lichen is a unique growth that is made up of a fungus and either green algae or bacteria called cyanobacteria. The algae or bacteria live underneath the sheltering fungus, and provide it with sugars (food) from photosynthesis. In return, they receive protection from the sunlight and are able to live without drying out. In this way, lichen represents one of the most unique living "partnerships" on Earth. Lichen is a hardy plant that grows on all kinds of bare surfaces in extreme environments, such as rocks, tree trunks, desert sand, cleared soil, and living bark. Most kinds grow very slowly—often less than 1 millimeter per year.