In May 1951, one year into the Korean War, PFC Francis P. Wall and his regiment found themselves stationed near Chorwon, about 60 miles north of Seoul. As they were preparing to bombard a nearby village with artillery, all of a sudden, the soldiers saw a strange sight up in the hills—like “a jack-o-lantern come wafting down across the mountain.”

What happened after—the pulsing, “attacking” light, the lingering debilitating symptoms—would mystify many for decades to come.

As the GIs watched, the craft made its way down into the village, where the artillery air bursts were starting to explode. “We further noticed that this object would get right into...the center of an airburst of artillery and yet remain unharmed,” Wall later told John P. Timmerman of the Center for UFO Studies in a 1987 interview. Suddenly, the object turned, Wall said. And whereas at first, it had glowed orange, now it was a pulsating blue-green brilliant light. He asked his company commander for permission to fire at the object with armor-piercing bullets from an M-I rifle. As the bullets hit the body of the craft, he recalled, they made a metallic “ding.” The object started behaving still more erratically, shunting from side to side as its lights flashed on and off.

Wall’s recollections of what happened next are stranger still. “We were attacked,” he said, “swept by some form of a ray that was emitted in pulses, in waves that you could visually see only when it was aiming directly at you. That is to say, like a searchlight sweeps around and the segments of light...you would see it coming at you.”

He remembered a burning, tingling sensation sweeping over his body, as if he were being penetrated. The men rushed into underground bunkers and peeped through the windows, watching as the craft hovered above them and then shot off, at a 45-degree angle. “It's that quick,” he said. “It was there and was gone.”

Three days after the incident, the entire company of men was evacuated by ambulance, with special roads cut to haul out those too weak to walk. When they finally received medical treatment, they were found to have dysentery and an extremely high white-blood-cell count. “To me,” says Richard F. Haines, a UFO researcher and former NASA scientist, “they had symptoms that sounded like the effects of radiation.”

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Was it an experimental new Soviet weapon?

In the wake of the Korean War, which ended in July 1953, dozens of men have reported seeing similar unidentified flying objects over the course of the 37-month conflict. The craft often resembled flying saucers. According to unofficial reports, as many as 42 were corroborated by additional witness reports—an average of more than one a month in just over three years.

At first, according to Korean war historian Paul M. Edwards, many researchers believed that the sightings were Soviet experiments, based on German technology and foreign research in anti-gravity. “These were supposedly so large they could carry 50 tons of weight and were powered by electromagnetic propulsion,” he writes in Unusual Footnotes to the Korean War. “What was being sighted, it was suggested, were discs the Russians were testing over the Korean skies.” But in the years since the fall of the Soviet Union, a number of Soviet reports of sighting UFOs over Korea have trickled in, discrediting these theories.

Why were there so many UFO sightings throughout the Korean war? Were they the product of thousands of exhausted men under incredible stress—or a sign of something more mysterious? From 1952 until 1986, the United States Air Force ran Project Blue Book, a systematic study into unidentified flying objects and their potential threat to national security. When it was shuttered, in December 1969, the Air Force announced they had found nothing of note, and terminated all activity under the auspices of the study.

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But many believe that the project ended abortively, and that there was more work to be done—leading to similar interviews with witnesses and other investigations being done by dozens of volunteers for decades after the project ended. Haines is one of them. He describes himself as a scientist with an open mind, rather than someone with something to prove. “I don't believe in them, I don't not believe in them,” he says. “I'm trying to let the data convince me one way or the other, which is the scientific approach.” But, he says, it’s striking how many accounts there are of similar sightings in the Korean War and other conflicts.

Other explanations?

An aerial view of the Korean DMZ in the Chorwon District, where Francis P. Wall saw the UFO. Yann Arthus-Bertrand/Getty Images

In the early years of the Cold War, it was often theorized that these crafts might be Soviet or Chinese vessels, with technology unknown to American troops. Haines believes this theory has been conclusively disproved.

“If they were,” he says, “they would have been building those crafts for use in later wars like the Vietnam War, for instance.” The Soviet UFO sightings Edwards describes make it similarly unlikely—as do the impossibly high-tech specifications of some of the sightings. In Wall’s case, for instance, he described a kind of force field taking effect a while after he began shooting, where his bullets simply ricocheted away from the craft.

Haines, for his part, believes the rash of sightings across the Korean war might suggest that something in the universe is especially interested in how human beings behave in the throng of military action. “We tend to be very creative to fight a war,” Haines says, listing off the various sciences and technologies that might come into play in military action. “If you were interested in how another country or another race of people fought their wars, you’d want to collect information on that, wouldn’t you?” He trails off. “That’s one possible explanation. There may be others.”

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But the vast majority of UFO sightings—as much as 80 percent—are later found to be totally ordinary phenomena, like clouds or human crafts, rather than anything otherworldly. In Wall’s case, precisely what he saw that day has never been conclusively proven or disproven. Without the testimony of other men in Wall’s regiment, it’s hard to ascertain whether they too had the same strange experience—, even if it can be corroborated that many did get very ill.

Why such long-lasting after-effects?

In the years following the war, Wall lost contact with many of the men in his regiment. After the experience, he remembered his company agreeing that they would not file a report, “because they'd lock every one of us up, and think we were crazy,” he told Timmerman. What made him choose to make a testimony, however, was the lasting after-effects of his illness, including permanent weight loss from 180 pounds to 138, stomach problems and periods of disorientation and memory loss after returning to the United States.

He retired in 1969, at the age of just 42, his daughter Renae Denny says, and spent 30 years out of work, struggling with the after-effects of the war. “Back then they didn’t know the name of it, but I guess you could say it was a form of PTSD,” she says. Over the years, he would tell and retell the tale of his strange UFO sighting. “The story was always the same,” says Denny. “It never changed through the years.” But there was other fallout: He was especially affected by the sounds of airplanes and once knocked his mother and sister to the ground after mistaking them for enemy troops. “I guess he would have flashbacks,” she says.

Wall’s recollections of the UFO sighting were consistent and acute. But whether what he remembered actually happened is harder to prove. Fighting conditions were almost intolerably stressful, and it’s entirely possible that he may have experienced some kind of hallucination, brought on by the terror of the situation, where he regularly feared for his life. It might also have been a moment of feverish delirium: Even the raised white-blood cell count that surprised army doctors, and Haines, is consistent with many of the bacterial infections which might also cause severe dysentery—as are hallucinations. In a later interview with Haines, Wall described how he had discussed what he saw with some 25 other men—but none ever came forward or could later be traced.

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In 2002, British researchers demonstrated a link between UFO sightings and Cold War hysteria—and pointed out how the number of sightings had nosedived as radar improved. “That cannot be a coincidence,” David Clarke told the Guardian. “Those early confirmations were just a product of a primitive radar system.” The flurry of UFO sightings Haines describes may have been the dual effect of these two threats: a potentially world-destroying war on the horizon, and the incredible pressure of being in the military.

Wall had experiences in those years in Korea that would scar him until his death in 1999. One night, Denny says, he managed to make his way through a pitch-dark minefield, praying for his life as he went. Others who made the same journey were not so fortunate. “When he went in [to the war],” she says, “he was happy-go-lucky, just a totally different person to when he came out.”

Whether the UFO sightings that Wall and so many other men reported were a product of this personality-altering trauma, or the effects of something requiring much greater investigation, remains a mystery.

WATCH: Full episodes of Project Blue Book online now.