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.
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.
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