in fallout 3 under the shack with the victory rifle is a cave.
its a glitch and you can go into the the mountain.
i want to call it kaelin's cave cause i found it.
if you want to see it go to the that shack, just out side the door is a few bords
with barals and tirers. jump down here till you land on a flat surface over looking a pond with a light fixture then just turn to the right and there it is