Another possibility that hasn't been mentioned yet is that it used to
be a cistern to hold rainwater. I don't think that was a common thing
to do, however, I have personally seen one cistern and have heard of
others in old houses.


That occurred to me, too, but if it was a cistern, they'd probably
have parged the walls, the floor would be either clay or some
kind of masonry, and you ought to be able to see where pipes for
the downspouts and kitchen pump used to be.