Donner Tunnels
There are some old train tunnels up near Donner Lake. They are abandoned and you can just walk through them and they go for miles. There's birds and bats in them and lots of graffiti. Parts of the tunnels are concrete, other sections have been carved out of rock.
We thought the temperature might drop a lot while inside the tunnels, but it really wasn't that cold inside the rocky tunnels and was pretty mild in the concrete tunnels. The first tunnel was carved out of rock and had water on the ground the whole way through and that one had an air-conditioned feeling the whole way through, but not cold enough to need a jacket. We could hear the bats squeaking as we walked through.