Aquatic frogs such as the leopard frogs (Rana pipiens) and American bullfrogs (Rana catesbeiana) typically hibernate underwater.
A common misconception is that they spend the winter the way aquatic turtles do: dug into the mud at the bottom of a pond or stream. Hibernating frogs would actually suffocate if they dug into the mud for an extended period of time. A hibernating turtle's metabolism slows down so drastically that it can get by on the mud's meager oxygen supply, but hibernating aquatic frogs must be near oxygen-rich water. They consequently spend a good portion of the winter just lying on top of the mud or only partially buried and may even slowly swim around from time to time.
In some terrestrial frogs, ice crystals form in places like the body cavity, bladder and under the skin, but a high concentration of glucose in the frog's vital organs prevents total freezing. A partially frozen frog will stop breathing, and its heart will stop beating. It will appear quite dead. But when its hibernaculum (the shelter where it spends the winter) warms up above freezing, the frog's frozen portions will thaw, and its heart and lungs resume activity. It’s pretty cool stuff.