UPDATE: You must physically visit this location to be able to log this as a find. Virtual visits are not allowed.
This park was originally the launch area for W-83, one of thirteen Nike anti-aircraft missile sites surrounding Washington, DC and Baltimore. The missiles and their launchers were located here; the target acquisition and tracking radars were about a mile to the northeast, off of Springvale Road.
W-83 was active from 1954 to 1962. As far as I can tell, none of the original structures from the launch area remain. However, what appears to be a protective dome for one of the radars from the IFC area has been installed here, with two of its panels removed so you can walk in and through it. The coordinates posted for this cache are, as near as I could place it, the center of the concrete pad on which the dome sits.
The parking area for this park is adjacent to (and possibly shares pavement with -- I couldn't tell) an elementary school, so be careful when school's in session. There may be another way in that I didn't see.
To claim this virtual cache, find the big sign for this park near the cache coordinates. (I've uploaded a picture of it with the cache answer blurred out.) To the right of the big sign are two smaller signs, one over the other. Email me the phrase on the bottom sign. Do not post the answer here, even if encrypted.
For more information on Nike missile sites, http://www.ed-thelen.org contains more than you will probably ever want to know. Many thanks to Ed Thelen for that site, without which I never would have gone looking for this one.