A few weekends ago Andy and I went hiking in Marin headlands. There is an amazing trail up a hill where all sorts of old military junk is slowly decaying.
The view of the backside of the golden gate is pretty amazing with the green hillside.
A few rooms were built into the hillside to be hidden from the enemy, covered over by dirt and grass. Over the years they've collapsed in on themselves, but some tunnels are still visible.
The best part of the hike was an abandoned missile site at the top of the hill. There are all sorts of abandoned buildings with weird pipes and electricity boxes. In the middle a huge elevated concrete platform has begun to collapse sideways. awesome.
There was some amazing grafitti as well. My favorite read "911 was an inside job!".