Passage is passage, underground or not. Very nice shot and timing. And us not knowing what it is makes it that much more intriguing. This image is one that I would call "open", allowing the viewer to supply the "missing" elements and interpret it. The opposite side is an image that is "finished" or "closed", where the image-maker presents it all, and does not allow any ambiguity or addition by the viewer. I personally find "open" images to be much more interesting and engaging.