Running wire in wall between floors
It boggles my mind when folks ask questions like this. How is anybody on the internet, supposed to know what's in your wall? We don't know where you live, what your house is made of, the layout of the house, "10 inches down" from what, etc.  Any answer somebody provided would be a guess, at best.

The only way to know for sure what it is, is to open up the wall and look.



If it's ~10" down from the second story floor, it's quite possibly the top plate of the wall below (that would assume 10" or 12" joists). Unfortunately, there's no way to guarantee that without looking. Happy drilling Bruh!