"john smith" wrote in message
...

The regs say you must communicate, have permission, a clearance, have a
transponder, etc to fly in any Class B/C/D Airspace.
The airspace is generally defined by the airport reference point altitude
and location on the field. For example the definitions state "from the
surface to..."
What requirements are there for flying BELOW the airport surface?
I know several airports with the requisite airspace that are located above
surrounding terrain where it would be possible to fly below the airport
surface.
What say the group?


The boundary of the airspace is generally defined by the airport reference
point but the altitude at that has nothing to do with the airspace. The
lower limit of a surface area is the surface of the Earth, you'll find it
quite difficult fly below the surface.