USAF Air Combat Command's "abandoned" Over the Horizon - Backscatter radar facilities in Christmas Valley, Oregon (transmit facility) and southeast of Tulelake, California (receive facility). The control facility was at Mtn. Home AFB in Idaho.
I live about an hours' drive from the Tulelake facility, and it is impressive to come upon it in the woods. It is huge, consisting of three, 8000' x 65' high antenna grid complexes with a 154-acre ground screen enclosed in wooden fences, each with a central control building structure. The central structure is the main one, with a microwave tower and two huge satellite dishes pointed to the sky.
Although the facility still looks brand-new, it is in fact pretty much gutted inside the buildings from what I have read. Built to detect Soviet bombers and/or cruise missles approaching over the Pacific Ocean, the Air Force was handed the keys to the facility right as the cold war had ended and the facility was never brought to operational status. It had been tested a few times and was used for some weather monitoring tests by NOAA, but was shut down and put in "warm storage" or "caretaker" status in 1991, three months after the Air Force took possession from the contractor who built it. One man comes out to the facility a couple of days a week to keep things clean and orderly, and he does a great job of it, by the way, but the internal radar gear, computers and encryption equipment was removed within the last year or so.
Since 2005, the system has been on the chopping block with plans to dismantle it. But apparently it is still entangled in red tape over the environmental effects of removing it completely or partially or just leaving it alone. Locally, there are groups that would like to see it converted into a solar power generation station since it is in a good location for that and is tied into the main regional power grid.
There is a similar facility, that once covered the eastern seaboard, in Maine, which was built before this one and was operational for a time but is also due to be, if not already, dismantled.
Anyone wanting more information about these spectacular relics of the cold war can find it on this web site:
http://www.globalsecurity.org/wmd/systems/an-fps-118.htm