Sounds like you might have done the thing that we warn people in sanding. Go in the direction only of the grain and be careful when using heavy grit sandpaper when on a belt sander.
If you cannot sand this out with heavy grit belt sander, then you might consider removing the mantle and taking it into a carpenter that has a big planer to true the mantle again. It can then be re distressed to look old.
You might try a portable planer IF you are experienced in this. If not,you will make the problem "more" worse.
This can be fixed, that is not the problem. Your plan "B" will be the industrial planer. Until then, plan "A" will be your sanding until its clean. Sand in the direction of the grain, do not sand too much in one spot, make long sanding swaths to keep it all uniform. Lay off the strippers now.