With nearly 400 units, the National Park System harbors some incredible wonders. Some are easy to recognize, such as Old Faithful, the Everglades, and the Tetons. But every now and again you come across something that you stare at and scratch your head over.

Such as the subject of this week's Mystery Photo. Is it man-made? Is it natural? Is it organic? What was it used for? Is it one of a kind? Is it bigger than a bread box? And in which unit of the National Park System might you find it?

Tell us the answer.