Class B airspace. You don't play games there as they typically protect a large terminal airport. Surface means ground level within (I believe) 5 nautical miles (6000'/n.m.) of the location defining the airspace. That location does not have to be the center of the airport, it can be a navigational aid substantially offset from runway center. Class B airspace is generally tiered, sort of like an upside down wedding cake, going out up to 30 miles from the airport. It can also have approach corridors extending one or more areas out further. So if any part of the surrounding area drops below airport elevation the surface becomes what you are standing on for the location you are at. The purpose of the "surface" call is to provide sterile airspace for aircraft on the "final" leg of their approach or their initial departure segment. More often than not ATC provides heading vectors to arriving and departing aircraft, and those vectors always include an assigned altitude. Since they "own" that 5 mile radius they can assign altitudes at whatever level they want to as long as that altitude provides for minimum terrain clearance as defined by the Minimum Obstruction Clearance Altitude (MOCA) in the IFR SID's charts. If an aircraft on an instrument flight plan is "cleared for the visual approach" the pilot can descend to the Minimum Descent Altitude before reaching a final approach fix if that is what is needed to keep the airport in sight, as long as MOCA is maintained. MDA's for a many precision instrument approaches are 200' AGL, and the aircraft could be on a DME/GPS Arc "circle to land" approach...