Thoughts on how to fix a depression in the roof

The thing that scares me is the fact that a magnet won't stick to it. Unless there's Bondo an inch thick, it should at least stick a little. Makes me wonder what it was patched with... sheet aluminum and pop rivets?
Gotta get a look at it, even if you just detach one edge of the headliner and shine a flashlight in there to get a glimpse of what's going on- or a borescope through the courtesy light opening, as mentioned.