Thoughts on how to fix a depression in the roof

This isn't an "ordinary" dent. I have a round spot on the roof of my Duster that has settled about 1/4". The spot is about 2' in diameter. I suspect that whoever built the car used a lot of bondo there and it's pulling away from the skin. I ran a magnet over the depression and found no metal. I don't know the full history of the car so I can't really tell much more. I really don't want to have to replace the roof but it's looking like I may have to go that route unless one of you body experts has another idea.

EDIT: I fixed it with a strong magnet. I guess there wasn't any bondo like I first thought.

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