I refer to this as the 737 Bunker as there is half a Boeing 737 next to it, it is located on the Salisbury Plain.
Warning signs that X-Rays will be present when warning sirens sound and an odd bunker building with a look out tower. Could be used for military training - but the warning about X-Rays sounds ominous. When the red flags aren't flying on the Plain it "may" be safe for you to walk over to it and have a look. It seems to be above ground. I have no evidence there is anything underground. Signs outside say it is operated by "COTEC" who I believe are a weapons testing company.
OS maps seem to refer to this location as a "Shelter".