The biosphere, (from Greek bios = life, sphaira = sphere) is the layer of the planet Earth where life exists. It is made up of all that is living on earth, from the smallest bacterium to the largest whale. It includes between millions of species of plants, animals, and fungi, and also two kinds of single-celled living things: those without nuclei 'Prokaryotes' such as bacteria, and those with nuclei 'Eukaryotes' such as protozoans. The biosphere extends over the Earth's surface in a thin layer from a few kilometers into the atmosphere, in very cold environments, to the deep-sea vents of the ocean depths, in very hot environments. This layer ranges from heights of up to ten kilometers above sea level, used by some birds in flight, to depths of the ocean. These are the extremes; however, in general the layer of the Earth containing life is thin: the upper atmosphere has little oxygen and very low temperatures, while ocean depths greater than 1000 m are dark and cold.