The cave in Bulgaria, although naturally in the shape that you see, was "augmented" by the local Bronze Age population to create a shrine to, well, you know. It is known as the "womb cave". This had to do with a mother-earth/sun-child cult in that region dating probably from the Neolithic. At one point (and only one) during the year, a single shaft of sunlight passes through the entrance and falls upon an altar in the cave. And yes, it is intended to look exactly like the anatomical feature that you think it looks like.
