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.