We have just the situation you describe at Seattle-Tacoma Airport...it sits
on high ground adjacent to Puget Sound. On the sectional, just west of the
"all the way to the ground" portion of the Class B, there is a blue square
with "-30" in it, and the legend says "Minus ceiling value indicates surface
up to but not including that value." The surface it refers to is the water.
Ergo, the Class B goes all the way to the water, and you can't fly past SEA
at 500 feet above the water without a Class B clearance.

Paine Field's Class D does not have a similar graphic, but its airspace
extends over Puget Sound and I'm guessing that the same reasoning applies.
Note that AIM 3-2-5 says that Class D "generally extends from the surface to
2500 feet above airport elevation," thus making a distinction between the
surface and field elevation.

Bob Gardner

"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?