OK, I'm not a big fan of cold weather. I've lived most of my life in Wisconsin and show no signs of getting used to cold weather—in fact, quite the contrary. Last week, however, I had an excellent winter experience and I was even outdoors when it happened. For a very brief time, the lake was frozen enough to be safe and there was not yet snow on it.
Update: I contacted the Center for Limnology at the University of Wisconsin (the people who study lakes) and they sent me the following explanation which I think is fascinating.
Even materials sunken in the water hold gases within the material--especially with something porous like a cinder block. As the water cools, the object cools and contracts, and the gases are forced out through the material. The first bubbles are caught under the top layer of ice, then as the ice freezes downward, the next bubbles are frozen underneath the first, etc., creating a layered effect. Of course, the longer something is submerged, the less gas it holds, however, even the rocks that have been in the lake for years still have gas within them. This year's freeze conditions resulted in some really clear ice, making the bubble/freeze effect even more defined.