To hibernate turtles outdoor, a pond 3 feet or deeper is generally recommended. The pond has to be deep enough so that it will not freeze through. If the pond freezes through, and the turtle with it, there is a good chance the turtle will not survive. There is some research going on about live freezing of fish and reptiles. It is known that baby painted turtles survive freezing in their first winter by producing anti-freeze in their bodies. The same mechanism has been found in several kinds of fish. However, turtles whose range does not extend north, in particular sliders, are not tolerant of freezing. Unless you want your turtle to become a scientific experiment, don't let it freeze in winter. Only hibernate a healthy, well-fed turtle. If the turtle is ill or very skinny, take it indoors for winter. There also has to be some mud at the bottom. Turtles dig into the dirt/mud/leaves at the bottom of bodies of water for hibernation. Also, the mud will protect them from freezing up to a point. It can also be too warm to hibernate your turtle outdoors. If the weather is too cold for the turtle to eat and move about much, but not cold enough for it to go into proper hibernation, it will use up too much energy and it will starve. The danger zone is around 50 Fahrenheit/10 Celsius. But watch your turtle. If it stops eating and doesn't move much for several weeks, but you won't get real winter where you live, take him inside until it gets warm again. Cool turtles are also less resistant to disease. The best way to decide whether to leave your turtle outside is to find out in which climate the turtle naturally occurs. Field guides usually include range information, and they are available at most libraries. Compare the turtle's natural habitat with your own yard. If there is an overlap, you can try outdoor hibernation. Also, make sure your turtle is the hibernating kind! Asian box turtles, for example do not hibernate and will die if left outdoors to freeze.