gbw - wet hole

The woodland walk around the boundary of our holiday site passed a couple of these deep holes. Not sure what caused them, but I don't think they are natural features - mainly the sides are too steep.
My suspicion is that they are bomb craters from WW2, as there were a lot of targets around here and as it's not in a field or on a road, it wouldn't have been filled in as a matter of urgency, and being in woodland, may well have been missed.