When I read the story, first I thought for sure that the fire was one more piece of evidence pointing to the existence of "Bad Hombres" -- people who use lonely mountain roads for shooting and drinking and littering and much, much worse. But then I started devising conspiracy theories.
Fact: there are a lot of people, including me, who don't want the Upper Gold Camp to reopen to motor traffic.
Fact: in order to open the road to traffic, the forest service would have to shore up the existing tunnels.
Fact: the easiest way to stop this project would be to collapse one of the tunnels and the easiest way to do that would be to burn the timbers.
So: it's not totally unreasonable for someone to set fire to this tunnel.
The authorities must agree: they are treating this as an arson. They've even closed down the area to look for clues.
We'll see what they find.