Q: Flying BENEATH Class B/C/D Airspace

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?