What is the open panel beneath the tailfin of the 747? It is at the same level as the APU exhaust. First time I noticed it as an SLF I was a little unnerved but it is so obvious that I assumed it would have been spotted during the walkround so obviously serves some function.
Here's a picture showing it clearly in the bottom left corner.
http://www.airliners.net/open.file?id=95518