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Liquid water is heaviest at about 4 degrees Celsius, not at the freezing point. As a lake cools toward winter, the 4-degree water sinks and the colder water stays above it. Ice then forms on the surface and insulates the water below. Fish are not surviving a miracle. They are living under a density quirk that puts the ice on top.

Water's odd density is why a lake freezes from the top and stays alive underneath.

Water Density · U.S. Geological Survey · Year unknown · Factapp