"The tortoise, like other reptiles, has an arbitrary stomach as well as lungs; and can refrain from eating as well as breathing for a great part of the year." --Gilbert White
I've just been learning about turtle "hibernation," or bromation. Lots of animals apart from those true hibernators can employ periods of deep sleep or vastly reduced metabolic activity. Freshwater turtles that live in parts of the world that freeze over have to go into such a state to survive the cold winter.
What they will do is usually drop to the bottom of a pond or lake. Unless frost line dips below the bottom of the pond the water will remain at 39 degrees F all winter no matter the temperature above because this is the temperature at which water is at its densest. So if it ever cools to below 39 degrees it will begin to float up and away from the turtle.
This might seem extremely cold but the turtle actually does just fine with it. The animal will also, as is the case with most forms of hibernation, lower its metabolism to an extremely low level. Many species' heart rates will fall to one beat about every ten minutes.
What I started wondering is: how do they breathe? Well they have specialized organs in their pharynx and near their anus that have a thin enough membrane that oxygen can be absorbed directly from the water. However, they are still unable to exhale. This means that lactic acid begins to build up in their muscle tissue. To compensate for that (and I think this is the most amazing part) calcium compounds from the shell will dissolve and enter their bloodstream, neutralizing the acid.
It's just wild to think, from a mammal's perspective, that all winter can be spent at the bottom of a lake, completely submerged in near freezing water. Evolution never ceases to amaze me.