It was an open secret what was going on at the American naval facility (NAVFAC) at Brawdy during the cold war in the sixties and seventies.

It was supposed to be hush-hush, but everyone seemed to know that the huge trench excavated down the valley into the sea at Penycwm was laid with cables connected to a system of underwater listening devices.

Ships were not allowed to anchor in St Brides Bay as they do today, in case their anchors fouled the sensitive listening gear.

However, the full extent of SOSUS, the American Navy Sound Surveillance System, remained under wraps until the early 1990s, according to a story in a recent edition of The Cape Gazette, an American newspaper circulating in the Delaware Cape region.

Milford Haven solicitor Michael Howells, a friend of NAVFAC deputy commander Ed Dalrymple, spotted the article.

SOSUS was a long-range early-warning listening system protecting America against the threat of Soviet nuclear attack.

Bases stretched from Nova Scotia to Barbados with additional sites on the Pacific coast and in Iceland and Brawdy, and this electronic ear' of the west stymied the Soviet submarine programme and gave the western allies a decisive edge.

Cables, some up to 100 miles long, were laid out along the ocean floor to the edge of the continental shelf. Each one had sensitive hydrophones attached to the end.

Naval personnel became so adept at interpreting the sounds they picked up, they could distinguish between crashing waves, whales, other ships and Russian subs.

The boast was that a Soviet submariner couldn't burp without being detected.

SOSUS played a key role in such incidents as the Cuban missile crisis and detecting sunken Soviet and Allied submarines.

The Russians embarked on a submarine quietening programme, but the Cold War was over before the programme could become effective.

However, the system continues to operate and has been used to track migrating whales and detect illegal driftnet fishing.

Ed Dalrymple said that normally there would be six or seven Soviet submarines on patrol in the Atlantic, but in summer there could be up to 100 when Russian war games took place.