Chilmark was the site of a major underground ammunition store since before World War II, built in a bathstone quarry site. By 1941 it seems to have evolved into an RAF bomb store. Rather than convert this existing protected site, a new surface bunker was built to house a Regional Government Headquarters (RGHQ).
The Home Office bunker known as RGHQ 7.1 was built on an adjoining site in the mid-1980s, to replace the former SRHQ at Ullenwood (near Cheltenham). It was one of the few purpose built RGHQs and became redundant within a few years of completion.
Along with most of the other RGHQs, it was closed in about 1992. It came onto the market in March 1997 and resold at least once since then. In February 2017 several arrests were made by Wiltshire Police after a huge cannabis farm was discovered within the bunker.