GeoGraph - photograph every grid square

For some inexplicable reason, this pool is not shown on the Ordnance Survey map. The picture has been taken from SO 85469 75614, looking north, and the pool is situated on the north side of the public footpath which runs roughly east to west between Heathy Mill Farm and Dunclent Farm. The obvious explanation for its non-appearance on the maps would be that it has only recently been artificially created, but to my knowledge, it has existed in its current location for at least seven years and probably much longer than that! Perhaps the next revised editions of the relevant maps will include the pool - or perhaps there is something lurking within its depths which render it invisible to aerial and satellite photography....!!