Last Saturday, Andre Poineau had a feeling there would be a good, hard freeze at his cottage along the shores of Lake Charlevoix near Boyne City, Michigan. But when he went down to the house to have a look the morning after a zero-degree night, he found the lake’s surface perfectly frozen, and eerily clear.

So clear, in fact, that he took a shovel with him to probe the ice as he went, since it lacked bubbles, sediment, cracks or any other clues to indicate where it might be thinner from one place to the next. He chopped a hole in the ice to see how thick it was: about 2 inches. So unnerved was he by walking out onto it