For the purposes of laying and maintaining a sewer line.
An easement gives one party the right to go onto another party's property. That property may be owned by a private person, a business entity, or a group of owners. Utilities often get easements that allow them to run pipes or phone lines beneath private property. Easements may be obtained for access to another property, called "access and egress", use of spring water, entry to make repairs on a fence or slide area, drive cattle across and other uses. The easement is a real property interest, but separate from the legal title of the owner of the underlying land.
