I moved into my home in May 2013 and had the Oak Flooring sanded and re-treated as it was a bit stained from previous owners (the flat is only 5 years also thought so the floor isn't very old at all).
Since the summer, these black marks have started appearing all over it, the worst appear to have cracks/holes in them. At first I thought it might me the knots in the wood but having kept an eye on them coming its not as they are appearing in places where there are no knots in the wood.
I do have underfloor heating (water) and am starting to get concerned there may some moisture underneath causing these? What do you think?