This old railroad tunnel was, until recently, open as a part the John Wayne Pioneer Trail. At 2.3 miles long, it is totally cool to stroll through. It is straight enough that from one end you can (once your eyes adjust a bit for the darkness) see the far end, which appears to be a distant star in the middle of the darkness of the tunnel. Apparently recent inspections have shown risk of cave-in's, so the tunnel has been closed. I was disappointed at this, but the disappointment of not walking through it again was preferable to the possibility of experiencing a cave in.
It is quite cold inside the tunnel, and you could feel the frigid air mixing with ambient air far from the opening. As the tunnel air contacts the outside atmosphere there is instant condensation, forming the mist you see here in the sunlight.
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