Well, Douglas, the laws of evidence change when it comes to an epic Al CiaDuh attack on the blessed homeland.
I guess the giant airliner must have come down at such a steep angle that it all disappeared under the dirt. What else could explain the complete lack of visible aircraft remains? It's underground, of course.
Of course there's no "buried plane pit" at the Pentagon. The giant airliner simply vanished. Not a shred could be seen. Not even an engine, or a sheered off wing, or any pieces of the plane's bulkhead.