A sewer is an artificial conuit (or pipe) or system of conduits used to carry and remove sewage (human liquid waste) and to provide drainage. In the 21st century developed world, sewers are usually pipelines that begin with connecting pipes from buildings to one or more levels of larger underground horizontal mains, which terminate at sewage treatment facilities. Vertical pipes, called manholes, connect the mains to the surface. Sewers are generally gravity powered, though pumps may be used if necessary.