In late September 1952, only months after a rash of “flying saucer” sightings over Washington, D.C. made headlines around the world, dozens of military officers participating in NATO exercises in the North Atlantic were struck by their own UFO fever.

Exercise Mainbrace was the largest peacetime military exercise since World War II. The war-game-style maneuvers simulated NATO’s response to a mock attack on Europe, presumably by the Soviet Union. The Mainbrace operation involved 200 ships, 1,000 planes and 80,000 soldiers from multiple NATO countries—including large deployments from the United States and the United Kingdom.

In a year dominated by news reports of UFO sightings, Pentagon officials half-joked with Naval Intelligence that they should keep an eye out for aliens during the NATO exercises, said Edward Ruppelt, the U.S. Air Force captain in charge of the top-secret Project Blue Book UFO investigations.

As it turns out, they weren’t off base. “[N]o one really expected the UFOs to show up,” Ruppelt wrote in his 1956 book, The Report on Unidentified Flying Objects. “Nevertheless, once again the UFOs were their old unpredictable selves—they were there.”

The first Mainbrace encounter came on September 13 when the captain and crew of a Danish destroyer spotted a triangular-shaped object moving through the night sky at alarming speeds. The unidentified craft emitted a blue glow and was estimated by Lieutenant Commander Schmidt Jensen to be traveling upward of 900 miles per hour.

On September 20, an American newspaper reporter named Wallace Litwin was aboard the USS Franklin D. Roosevelt, an aircraft carrier participating in the Mainbrace exercises, when he saw a commotion on deck: several pilots and flight-crew members pointing at a silver sphere in the sky that appeared to be following the fleet. Litwin quickly shot four color photos of the round object, which he assumed was a weather balloon.

In a letter to a UFO investigator years later, Litwin recounts that he went below deck and joked with fellow newspaper correspondents that he had just “shot a flying saucer.” This caught the attention of the ship’s executive officer, who informed Litwin that no weather balloons had been released that day. The officer then radioed the Midway, the only other ship in the vicinity, which also confirmed that no weather balloons were in the air or unaccounted for.

“In other words, the skies above this NATO fleet were very carefully observed and nothing flew around overhead unobserved,” wrote Litwin, “But I knew that I had taken a picture (4) of what looked like a ping-pong ball 10 feet over my head.”

Ruppelt and the Project Blue Book team followed up with the Navy and interviewed members of the flight-deck crew. Some dismissed it as a weather balloon, while others had their doubts.

“It was traveling too fast, and although it resembled a balloon in some ways,” wrote Ruppelt. And “it was far from being identical to the hundreds of balloons that the crew had seen the aerologists launch.”

The most perplexing sighting—the one that may have single-handedly relaunched the British military’s interest in UFOs—was reported by a half-dozen Royal Air Force (RAF) officers and air crew based in Topcliffe, Yorkshire, England.

It took place on September 19, as a British Meteor fighter jet was returning to the Topcliffe airfield from exercises over the North Sea. When the plane had descended to 5,000 feet, crew on the ground spotted a silvery, circular object traveling several thousand feet above the Meteor, but on its same trajectory.

In a report preserved in the National Archives, RAF Flight Lieutenant John Kilburn of 269 Squadron said the object then began to descend toward the Meteor, “swinging in a pendular motion…similar to a falling sycamore leaf.” At first, Kilburn thought it was a parachute or engine cowling that had broken loose from the jet.

Then the object stopped suddenly in mid-air, rotated on its own axis and zipped off at incredible speeds over the horizon.

“The acceleration was in excess of that of a shooting star,” reported Kilburn. “I have never seen such a phenomenon before. The movements of the object were not identifiable with anything I have seen in the air.”

Unlike previous UFO sightings kept hush-hush by the RAF and Royal Navy, the Topcliffe sighting was leaked to the press—and splashed across the front page of Sunday newspapers. “‘Saucer’ Chased RAF Jet Plane,” reported the Sunday Dispatch with a photo of five of the airmen, including Kilburn.

The circus-like publicity surrounding the Topcliffe incident put the British military intelligence in a difficult spot. They couldn’t ignore questions from the press, but they also weren’t interested in a serious investigation into UFOs. They’d already been down that road.

While conducting research in the UK National Archives in 2001 for a book called Out of the Shadows: UFOs, the Establishment & the Official Cover-Up, British journalist and UFO investigator David Clarke made an incredible discovery. Despite officials’ repeated denials that they existed, he uncovered documents that referenced top-secret UK government UFO investigations.

The six-page report from the Ministry of Defence’s Directorate of Scientific Intelligence (the equivalent of the CIA in America), dated June 1951, was produced by a top-secret panel of military-intelligence experts known as the “Flying Saucer Working Party.”

According to the report, the five-member team had been meeting since 1950 to analyze reports of unexplained sightings from RAF and Royal Navy pilots. The Flying Saucer Working Party, much like the Air Force higher-ups overseeing the Project Blue Book investigations in America, dismissed all sightings by experienced military personnel as either “mistaken identification of conventional aircraft,” “optical illusions and psychological delusions,” known “astronomical or meteorological phenomena” or “deliberate hoaxes.”

The clandestine team concluded that the only way to get substantiated data on UFOs would be to establish a global network of radar stations and photographers continuously monitoring the sky for aberrations.

“We should regard this, on the evidence so far available, as a singularly profitless enterprise,” they wrote. “We accordingly recommend very strongly that no further investigation of reported mysterious aerial phenomena be undertaken, unless and until some material evidence becomes available.”

This was the conclusion shared with Winston Churchill when he fired off a memo in the summer of 1952 reading, “What does all this stuff about flying saucers amount to? What can it mean? What is the truth? Let me have a report at your convenience.” Churchill was shown the top-secret report and the topic of UFO investigations was briefly laid to rest. That is, until Exercise Mainbrace.

In the wake of the Topcliffe sighting and resulting newspaper coverage, the British military intelligence was forced to “officially recognize the UFO,” according to Ruppelt of Project Blue Book. In 1953, the British Air Ministry established a “UFO desk” within the Deputy Directorate of Intelligence known cryptically as “AI3.” From then on, all unexplained sightings by British military personnel would be controlled internally, classified as “restricted” and not shared with the press.

Clarke, for one, isn’t surprised that dozens of sailors and airmen spotted unidentified and unexplainable aerial phenomena during two weeks of high-stakes exercises.

“You have all these military personnel on high alert looking for potential intruder aircraft,” he says. “There’s a good chance they’re going to see things that might have otherwise been ignored.”

As to the seriousness of the British military’s investigations into Topcliffe and later UFO sightings, Clarke cites a newspaper clipping published months after the Mainbrace exercises where a reporter pressed an Air Ministry official for the results of their investigation. The official said he had “no idea” if the investigation was ongoing or if its conclusions would be shared with the public.

“Was there any chance that it might turn out to be a flying saucer?” wrote the reporter. “One gathered from the low chuckle of the official that there was not the remotest chance. ‘We take those stories with a large spoon of salt, old boy,’ he said.”