I pass this site every day on my way to and from work. Curiousity finally got the better of me and I started asking some old-timers about it. I've been told it was called the "C-Sash mine", it was part of an old mill, and a few other odd answers. Finally asked a rancher who was feeding across the street and he said it has been used for various things over the years, but he always knew it as "Cobalt Mine". Since Cobalt is one of my favorite colors, I decided to take his version and run with it.
Cache is not on private property, not inside the fence line, and you will have to park about 25 feet before GZ if you're heading north. If you're heading south, I suggest turning around, or be prepared to walk a bit. This road gets a lot of foot traffic with dog walkers and joggers, plus plenty of vehicle traffic, so please be careful both for safety sake and stealth.
Also, be aware that you will have to cross a narrow creek. I have long legs and did it in my tennis shoes, but if you're short or worry about wet feet, you might want to haul your boots along. The container is a camo'd finger paint jar with room for a few small trading items only. Small TBs/coins will be fine there too. The hiding spot is pretty obvious on approach so please be sure to cover it well when you put it back.
Cache on!