When a massive international war-games exercise known as Operation Mainbrace convened in the North Sea in 1952, it brought together 80,000 military personnel, 1,000 planes and 200 ships from nine countries. There were also some unexpected attendees: UFOs .

Multiple sightings of unidentified flying objects (UFOs) during Operation Mainbrace (also known as Exercise Mainbrace) were documented by pilots and naval officers and appeared on radar, according to the National Investigations Committee on Aerial Phenomena (NICAP), a federal agency that compiled witness accounts of UFO encounters from the 1950s through the 1980s.

This still-unexplained incident inspired the Season 2 finale of the History channel's "Project Blue Book," the dramatic series about the U.S. Air Force program — also named Project Blue Book — that investigated UFO sightings from 1952 to 1969, when tensions from the Cold War were at their highest. Airing tonight (March 24), the episode leads UFO hunters Dr. J. Allen Hynek (Aidan Gillen) and Capt. Michael Quinn (Michael Malarkey) into a tense naval standoff, in which the appearance of unknown flying and diving UFOs nudges edgy military commanders toward the brink of war with the Soviet Union.

But what, exactly, was Operation Mainbrace, and what really happened there?

Related: 7 things most often mistaken for UFOs

In the fall of 1952, the North Atlantic Treaty Organization (NATO) , an international military alliance, staged a 12-day exercise in waters near Norway and Denmark. Dubbed Operation Mainbrace, the exercise brought together naval forces from nine countries, with most of the might represented by the navies of the United States and the United Kingdom. It was "the largest and most powerful fleet that has cruised in the North Sea since World War I," The New York Times reported on Sept. 27 of that year.

"The whole point of Operation Mainbrace was to flex NATO's force and show the Russians that we would be prepared for a battle at sea," David O'Leary, "Project Blue Book" creator and executive producer, told Live Science.

"Tensions were already high," O'Leary said. "This was a training exercise, but it was also a flex of muscle. And then, in this exciting and tense situation, there were these unexplained UFO events."

A roaring Arctic gale and high surf hindered some of Mainbrace's planned maneuvers, according to the Times. And then, there were the UFOs.

"Someone in the Pentagon had half-seriously mentioned that Naval Intelligence should keep an eye open for UFOs, but no one really expected the UFOs to show up," Capt. Edward J. Ruppelt , a U.S. Navy officer and director of the U.S. Air Force's Project Blue Book from 1951 to 1953, said in his account of the Mainbrace sightings.

"Nevertheless, once again the UFOs were their old unpredictable selves — they were there," Ruppelt added.

"A silvery, spherical object"

On Sept. 13, 1952, several crew members on the Danish destroyer Willemoes saw something unusual: "an unidentified object, triangular in shape, which moved at high speed toward the southeast," according to NICAP records . It glowed with a bluish light, and the destroyer's commander estimated its speed at more than 900 mph (1,448 km/h).

More sightings took place over the next week. On Sept. 20, 1952, three officers with the Danish Air Force spotted "a shiny disk with metallic appearance" flying overhead and vanishing into the clouds. Also on that day, personnel onboard the American aircraft carrier USS Franklin D. Roosevelt spied "a silvery, spherical object" that traveled across the sky, NICAP records show . A reporter named Wallace Litwin, who was on the aircraft carrier, described the UFO as resembling "a white ping-pong ball." Litwin allegedly captured photographs of the object that were reviewed by U.S. Navy Intelligence officers, but the images have never been released to the public, according to NICAP.

Three aircraft carriers proceed full steam ahead during the NATO exercise "Mainbrace" in the North Sea, in September 1952. Left to right: USS Wright, HMS Illustrious and HMS Eagle. (Image credit: he Royal Navy/Imperial War Museums)

The next day, pilots with the British Royal Air Force noticed a UFO — "a shiny sphere" — as they flew their jets in formation over the North Sea.

"When returning to base, one of the pilots looked back and saw the UFO following him. He turned to chase it, but the UFO also turned and sped away," according to the NICAP report.

None of the Mainbrace UFO sightings were ever explained, NICAP says.

Fear of the unknown

More recently, U.S. Navy pilots documented UFO sightings during training exercises in 2004 and 2015; allegedly declassified video footage of fast-moving "unidentified aerial phenomena" was published by The New York Times in December 2017 and in March 2018, Live Science previously reported .

While UFO sightings can be unsettling, a bigger threat may emerge from the uncertainty and fear that UFOs inspire, O'Leary told Live Science. During an operation such as Mainbrace, any unexplained aircraft sightings could have been interpreted as an attack by a hostile enemy. While there are no records of any such conclusions during the real Operation Mainbrace, the "Project Blue Book" season finale features a U.S. Navy admiral who interprets the UFOs as a Soviet threat that must be answered with deadly force.

"Is the most dangerous thing the mystery up in the sky, or is the true danger humanity's reaction to the unknown?" O'Leary said. "When we face a new threat we don't understand, oftentimes we're gripped by fear. We make decisions that we think are right for us at the time, that may not be, because we're not able to put our own terror of the unknown into perspective," he said.

"Operation Mainbrace," the Season 2 finale of "Project Blue Book," airs March 24 at 10 p.m. ET/PT. Watch an exclusive excerpt from the episode below.

Originally published on Live Science .