How far down does an easement go?

An easement is a limited right to use land for a stated purpose, not ownership of the land. A utility easement for a buried line lets the holder place and service that line. It does not hand over an unlimited column of earth below your lot.

Courts read the scope from the document, and when it is silent they usually allow what is reasonably necessary for the purpose. Because of that, how far down an easement goes depends on what was granted and what the use actually requires, not on a fixed depth.

Minerals are a separate question. An ordinary utility easement does not transfer oil, gas or other minerals, which are owned and leased on their own terms. Check the grant for language about the subsurface, and check your title for any separate mineral reservation.

What this looks like in practice

A water main easement lets the city lay, repair and replace its pipe in a strip across your yard. The city may dig down to the pipe and enlarge the trench when it repairs it. It cannot, because of the easement, claim the ground below the pipe or use it for something unrelated.

Overhead lines work the same way in the air. The easement covers the clearance the line needs, not everything above your lot. That is why a tall tree or a new roof edge near a wire becomes a question about the easement's stated purpose and clearance.