What is a subsurface easement across one's property for a sewer line best described as?

A subsurface easement is a right to use the area underneath one's property for a specific purpose, such as installing a sewer line. An encroachment is when someone builds on or uses a portion of someone else's property without permission. This is not the case for a subsurface easement, which is granted with permission for a specific purpose. An easement by necessity is a type of subsurface easement that is granted when it is necessary to access a property, but this does not apply to a sewer line. A personal easement in gross is an easement that is granted to an individual or entity, rather than to the land itself. Again, this does not apply to a sewer line, which is typically granted to a utility company or municipality. Therefore, the best description for a subsurface easement across one's property for a sewer line is an easement in gross