The zone, including the earth's surface (and surface water), the adjacent atmosphere, and the underlying crust, where life can exist. The earth is usually considered as having three spherical zones: the solid lithosphere, the liquid hydrosphere, and the gaseous atmosphere. The biosphere encompasses parts of all three of these zones. Its limits are not well defined; most living organisms exist in a region extending about 100 m into the atmosphere and about 150 m below the surface of the oceans. The term can also include parts of the earth and atmosphere that depend on the present or past existence of living organisms, e.g. coal deposits or atmospheric oxygen. The biosphere can be regarded as a single *ecosystem
and is often called the ecosphere.