Water watching: frozen edition

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.

The ice was incredibly smooth and the clearest lake ice I've ever seen. We put on our ice skates and special skate covers that Mr. Rududu designed to keep our feet warm. (They really work.)

I loved the reflections but the most wondrous thing was down below us—something I'd never seen before. We were skating where the water is quite shallow, less than a yard/meter. The ice and water were so clear we could see the sand on the bottom. Anywhere there was an object on the lake bottom, such as a rock, there was a 3D "ghost" of bubbles in the same shape above it in the ice. Here are the bubbles over a cement block.

Here are the bubbles a sunken wooden object made up of boards.

 I have no idea why this happened, but it was lovely to look at. (If anyone has an idea, please let me know!) Now it's covered up by snow. I'm so glad I went out to see this.
  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.