What is the purpose of an underground fence?

You may be more familiar with the term "invisible fence." An underground and invisible fence are the same thing. Basically it's a buried wire around your property that communicates with your pet's collar. If they pass over that line they get buzzed and generally stop.

An undergrond fence in beneficial in training pets to stay in the yard. The fence is invisible to your neighbors but will emit a slight shock if your pet is too close and attempts to leave the designated area.