Last atom of bomb secrecy.
Prime Minister Harold Wilson had gone to extraordinary lengths to prevent news of the test at the US underground facility in the Nevada desert on May 22, 1974, leaking out.
But he was forced to come clean after veteran Daily Express reporter Chapman Pincher learnt about it... in a dream.
The unlikely tale was recounted to Foreign Office official Crispin Tickell by Express foreign editor John Ellison.
"Mr Ellison said that, believe it or not, Mr Pincher's source was Mr Pincher," Mr Tickell wrote in a memorandum to No.10.
"He was on a fishing holiday in Scotland. One morning he said that he had had a particularly vivid nightmare about a nuclear explosion which he was sure was British.
"His companions told him that he was obsessed and should get back to his fishing."
Eventually Mr Wilson (pictured) was forced to admit it was true.