Freediver Peter Kapoun went for a swim under the ice of frozen Lake Milada in the Czech Republic. His swim looks elegant, graceful, and cold as hell.
I suppose this counts as a polar bear plunge.
Freediving is the act of swimming underwater without breathing assistance, so it only makes sense to take the activity into freezing cold water under ice.
Kapoun swam on his back and then flipped to his stomach and when he popped out of the ice, assured his camera operator that he was okay.
Don't try this at home... or any home that has a layer of ice on top of a body of water. We assume Kapoun is an expert.