The tree tunnel entrance is a tree stump located in the forest near Stalag 13. The prisoners uses it to leave the camp to perform their missions and then to reenter the camp without being seen by the guards.
The tree stump, from outside, at first appears to be a cut down tree. But upon closer inspection, it is shown to be a hollowed out stump, the top of which is opened up to allow a person inside. The person would then enter (or exit) the stump and climb down (or up) a ladder that would lead to (or from) the main tunnel.