Opening in 1973 and closing in 1995, the US Navy base, next to RAF Brawdy in Pembrokeshire, was officially an "Oceanographic Research Station".
But it has long been know that it was a processing centre for a network of underwater microphones, some of which were submerged off the British coast, listening to submarine movements in the Atlantic.
The 7.6 acre NAVFAC (Naval Facility) has since been turned into the Brawdy Business Park, with office and workshop space being rented out in the former admin blocks of the base.
The 58,000 sq foot 'Terminal Building' was built to minimise static throughout.
This was a major intelligence facility with 22 US Navy Officers, 278 other ranks and 7 civilians working there in 1980.
Its budget was $20m between 1978-1984, 80% of which was spent on electronics or computer equipment.
$3.6m was spent on doubling the size of the operations block in 1981.
Some have suggested the hydrophones it used were located in St. Brides bay (the large semi-circular bay on which Brawdy is located), others suggest (and a look at a map appears to agree) that on their own, hydrophones in St Brides bay would have a limited 'field of view' (due to Ireland blocking off most of the north Atlantic) and that Brawdy was the landing site of a much larger network that stretched hundreds, if not thousands of miles out into the Atlantic.
It covered the areas through which all Atlantic Soviet submarines would pass on their way towards the US coast.
Little is known for certain about the base, though details of its role may have partially been revealed in 1993 when a whale researcher obtained recordings of whale song from the base, he was told that once, US Navy technicians were able to track an individual whale for 40 days by which time it had reached Bermuda - rather impressive technology.
The US area is sited next to the Brawdy airfield which was originally a WWII RNAS station, then an RAF base and since January 1996 an Army camp: Cawdor Barracks.
The 14th Signal Regiment (Electronic Warfare) are housed there, though it would appear to just be coincidence that a regiment who would benefit from Elint facilities have occupied the site.
A letter on behalf of the Commanding Officer of the Regiment states "we do not utilise anything that may have been left by the US Navy and I doubt very much that they left anything anyway".
The Navfac was included under the RAF station bye-laws, despite being geographically separate. Although only several hundred metres from the British base, relations were not always that good:
The fire alarm system of the Navfac was linked to that of RAF Brawdy, it is reported that on at least one occasion when the British fire control team raced round to help the Americans they were told at gunpoint, that they did not have permission to enter, and that the US personnel would deal with it themselves.
The base is discussed in some depth in a UFO book by Peter Paget - 'The Welsh Triangle', and seemed to have an unusually great suspicion of vast underground areas attached to it. But no underground areas or protected accommodation are to be found.
The site was subject to several anti-nuclear protest actions, the biggest of which being a march by 'Women for Life on Earth' (who formed the nucleus of the Greenham Women) from Cardiff in 1982.
The Welsh Triangle By Peter Paget. Published by Granada Publishing Ltd in Panther Books 1979.
When frequent sightings of UFO activity were recorded along a secluded portion of the Welsh coast, author, Peter Paget decided to investigate.
What he found was alarming evidence for the existence of an alien base beneath the rocky verge of the Atlantic, the frightening possibility of extra terrestrial life beneath the sea - and an official cover-up which has serious implications for the future.
Pagets story begins in February 1977 when the first of many sightings was reported in local papers.
Some of the reported sightings are as follows:
February 2, 1977: Penarth, Cardiff: Billy Price and several other boys saw a white light moving at very high speed about ten metres above the ground.
February 4, 1977: Huberston School, Milford Haven. A group of twenty children sighted a cigar shaped UFO hovering overhead during their lunch break.
February 11, 1977: Rhosybol School, Anglesley. Nine girls and their teacher saw a cigar-shaped UFO hovering over the school. It was a sunny afternoon with perfect visibility.
February 16, 1977: Pembroke Dock. Graham Howells sighted a bright silver, metallic object, hovering over his school.
February 17, 1977: Haverford West. Schoolboy Mark Jones and a friend saw a UFO hovering near the towns Grammar school. The boys watched the object from about 30 metres away and said that it was 15-ffot long, orange cigar-shaped fuselage, carrying a blue flashing light.
October 31, 1977: Aberdare. Mr P.D May reported that his two sons were terrified by a huge, noiseless UFO. It moved toward the children and two of their friends before continuing on.
Mark Barton of apra.org.uk recently mentioned 'Nocturnal Booms' heard in the mid 1970s, with relation to sightings of delta and triangle craft.
UFO journals did mention the booms, notably Paget's Fountain Journal.
It was in its pages that a reference was made to research at Bristol University to establish a cause.
In 1993 apra.org.uk managed to make contact with one of the original researchers, but he was disinterested, and was dismissive that it was 'merely Concorde' over the Atlantic.
The booms did get one mention in a UFO report in the BUFORA Bulletin, and were featured at least once on News at Ten TV bulletins.
The obvious point to make is that Concorde still flies today, and the booms are not heard on a remotely regular basis. Also military aircraft regularly go supersonic off our coasts, yet they do not propagate such booms inland.
Mark Barton remembers reports of booms and rumbles over America during alleged flights of 'The Aurora' and other such aircraft.
Were these booms made by UFOs or experimental aircraft – we still do not know!
another book in 1980, "UFO-UK" (157pages) - New English Library, London.
Speakers: (Confirmed) Brian O'Leary, David Percy, Bill Holden, Matthew Williams, Dea Martin, Simon Lewis, Peter Paget.