A manhole is the top opening to an underground utility vault used to house an access point for making connections or performing maintenance on underground and buried public utility and other services including sewers, telephone, electricity, storm drains and gas. It is protected by a manhole cover, also known as a 'biscuit', a plug designed to prevent accidental or unauthorized access to the manhole. Those plugs are usually made of metal or constructed from precast concrete (especially in Europe). Manholes are usually outfitted with metal or polypropylene steps installed in the inner side of the wall to allow easy descent into the manhole.

Manholes are generally found in urban areas, in streets and occasionally under sidewalks. They are usually in circular shape to prevent accidental fall of the cover in the hole.

http://en.wikipedia.org/wiki/Manhole