'Project Blue Book' Explores the UFO Conspiracy Theory Surrounding "Operation Mainbrace"

History buffs and armchair ufologists really get the best of both worlds with the History Channel series Project Blue Book, even if experts in both fields have criticized its accuracy in some regards. Over its first two seasons, the series, led by Game of Thrones star Aidan Gillen, has explored the intersection of some of the Cold War and many unexplained flying objects sighted by military personnel throughout the 1950s and '60s.

The Season 2 finale is no exception as it dives into Operation Mainbrace, or Exercise Mainbrace, which is both a real NATO naval exercise and an incident cited by ufologists as the most impressive UFO incident ever reported. Here's what historians and Project Blue Book experts have to say about Operation Mainbrace.

The operation, officially named Exercise Mainbrace by the U.S. Air Force, occurred in September 1952 as the first official exercise carried out by the Allied Command Atlantic (ACLANT). ACLANT was one of the two main military arms of the North Atlantic Treaty Organization (NATO). The exercises occurred from Sept. 14-25, 1952 in the waterways and airspace surrounding Norway and Denmark.

The purpose of the exercises, carried out by nine international navies including the U.S. Navy, was to demonstrate to Norway and Denmark both their vulnerability to an attack by the Soviet Union and how NATO forces could converge to protect the Scandinavian countries from invasion.

It was an impressive show of force involving 80,000 military personnel, more than 200 ships, and more than 1,000 aircraft. The operation was regarded as a success in that it showed the might of the allied forces — to the Soviet Union as well as to Norway and Denmark. In fact, the exercise was regarded by the U.S.S.R. as war-like acts and potentially heightened hostility between the communist state and its adversaries.

With so many militaries converged in one place looking to the skies, personnel saw plenty of flying objects — like I said, there were over 1,000 aircraft involved in the exercises. But some of the aircraft observed didn't seem to belong to any navy or air force that had been invited to the party.

The objects these men observed in the sky seemed to have characteristics and capabilities no known aircraft possessed. Two days into the exercise, according to Mysterious Universe, there were at least two official reports of UFO encounters made by observers aboard ships in the Atlantic between Ireland and Iceland.

One cited a “blue / green triangle,” flying at around 1,500 miles per hour, an unprecedented speed in 1952. Chuck Yeager had only managed to break the sound barrier (767 mph) five years earlier, and the craft these personnel observed seemed to be traveling twice that fast.

Members of American forces also reported seeing flying objects of unprecedented speed, size, and capability during the exercises. Often the sightings had multiple corroborating witnesses, and ufologists claim no adequate explanation for what the men observed has been put forth.

Edward J Ruppelt, former head of the USAF's Project Blue Book and author of The Report on Unidentified Flying Objects, even claims the Operation Mainbrace sightings led the British Government to officially recognize the existence of UFOs. It took 'til 2019 for the U.S. to (sort of) acknowledge that they're real.

Check out the season finale of Project Blue Book to see how they portray Operation Mainbrace Tuesday, March 24, 2020 at 10:05 ET on HISTORY.