Most transformers serving customers from underground distribution lines are called "pad-mount" transformers. These are electrically the same as pole-top units, but are packaged in a box-like, oil-filled metal enclosure and installed on a ground-level concrete foundation, or "pad". Underground cable enters and exits the unit from under the ground and out of sight. There are no exposed live parts outside a pad mount transformer, so no fence or further confinement is required. The case or box is designed to be resistant to unauthorized or accidental entry. It is very safe even though it sits essentially on the ground.
There is one other type of commercially available underground transformer. This type is truly an underground device, for it is completely below ground level. It is designed for installation in a manhole or small underground vault. In general appearance, it resembles an overhead unit, but since it is underground, both high and low-voltage terminals must be fully insulated and waterproof.