The Mystery Door

There's a little boxed off section in the corner of the family room.  When we were painting this room, we noticed it had a bunch of little hinges going up left side, and a bunch of screws on the right side.

We're pretty certain that it contains the main drain stack from the upstairs bathrooms. 
Pretty clever to leave access for future issues.  You can see it in the background of this photo:

Flash made the paint
look really bright here!

Mike noticed a little leaking in the basement, and thinks it's coming from the stack (gross).  After much procrastination, the other night he decided to open up the secret door to see what's going on in there.

Here comes the mystery...

See that box at the TOP, where the corner meets the ceiling? 
It was built OVER the hinged door.

WHY in the world would you do that? 

We tried to pry it open, but I think we need to get the saw out to cut a line across the door, just under the box, so we can at least open it!

Now I'm really dreading what we find in there! 
Will post when we finally get it open...