The Royal Observer Corps was a civil defense organisation operating in the United Kingdom between 1925 and 1995. It was created to perform the tasks of the visual detection, identification, tracking and reporting of aircraft. During World War Two it performed these tasks with distinction as the new RADAR system could not operate successfully over land. In 1995, the advent of the cold war led to an additional task of detecting and reporting nuclear explosions and the resulting radioactive fallout. To enable this to happen, between 1958 and 1968, 1,563 underground monitoring posts were build approximately 8 miles apart.
This cache is located at the site of one of these monitoring posts. Further details, including photographs, can be found at http://www.subbrit.org.uk/rsg/roc/db/988374305.011001.html.
I found out about these sites during a visit to the Kelvedon Hatch secret nuclear bunker, just north of London. It is well worth a visit.
The cache itself is a small tube containing some log sheets and a couple of pencils. It can be done as a cache and dash as there is room to visit with your car, but please be carefull if arriving on a motorbike as the pull in is uneven and stony.