^^^ Mine is small and it's only 2 foot deep. In the winter, it would be one giant ice cube because there is no ground heat at all. Mine is not on/in the ground. The wind and cold are too much to overcome with a heater.
One more pic of them enjoying the artificial sun!

I almost think they are happier when moved inside!
Only 10 minutes allowed per turtle for tanning! The line forms to the bottom.