Driving by my neighbor's frozen over pond yesterday, I noticed a bullseye pattern in the middle. I pointed it out to my wife and she said she thought it was where the overflow pipe was, but that was at the end of the pond and had a simple smaller circle of no ice. The inlet is likewise not frozen, with a much larger area with no ice - the ducks are swimming there in the picture. This was in the middle of the pond. I'm just curious what would cause such a pattern of freezing.