I sanded down the high area, but the "wood" in that area doesn't sound right, sounds like tapping on balsa wood (see video).
Just a guess on my part, but I reckon when the holes were being drilled, they had an accident and patched this section up (hence no hole going all the way through), with what I do not know.
https://www.youtube.com/watch?v=hVMd...ature=youtu.be
In the same area, there is a greenish patch, could it be some kind of bog? Probably not the greatest place to have a soft spot.
https://s19.postimg.cc/hlphvczjn/20180620_182002.jpg