In February 1949, the Los Alamos, New Mexico Skyliner newspaper ran a piece on what it referred to, in typical newspaper parlance, as “flying saucers”—and a possible conspiracy around them:

“Los Alamos now has flying green lights. These will ‘o wisps seen generally about 2 a.m., have alerted the local constabulary and their presence is being talked about in Santa Fe bars. But local wheels deny any official knowledge of the sky phenomena. Each one passes the buck to another.”

The story ended with, “Have you seen a green light lately?”

In fact, a great many had, and would continue to do so—enough to prompt TIME magazine, in November 1951, to publish a piece on the phenomenon called “Great Balls of Fire.” What makes the multiple sightings of “flying green lights” in New Mexico in 1948 and onward such a significant chapter in UFO history is exactly that—there were multiple sightings.

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That was unnerving enough. But most alarming—particularly to the United States government—was that the sightings were concentrated around the Los Alamos and Sandia atomic-weapons laboratories. And other highly sensitive military installations, including radar stations and fighter-interceptor bases, weren’t far away. That meant the sightings were reported by typically cool-headed pilots, weather observers, scientists, intelligence officers and other defense personnel, and led many to suspect the fireballs were Soviet spy devices.

Like a meteor—but not

On the night of December 5, 1948, two separate plane crews reported having seen a “green ball of fire,” heading west to east. In one of these instances, the fireball raced head-on toward the plane itself, compelling the rattled pilot to swerve the plane out of the way.

One pilot, sometime later, would vividly describe the green fireballs: “Take a soft ball and paint it with some kind of fluorescent paint that will glow a bright green in the dark… Then have someone take the ball out about 100 feet in front of you and about 10 feet above you. Have him throw the ball right at your face, as hard as he can throw it. That’s what a green fireball looks like.”

When a crew of intelligence officers, led by Dr. Lincoln LaPaz, head of the University of New Mexico’s Institute of Meteoritics, plotted the fireball’s flight path and scoured the area a meteorite would have hit, they found nothing—no meteor fragments, no debris, no craters, no evidence of fire.

The inexplicable sightings continued in the area, with sightings on December 6th, 7th, 8th, 11th, 13th, 14th, 20th and 28th. December 20th proved a turning point, literally, and a particularly alarming one for those clinging to the theory that these were meteors: The balls of fire descended from the heavens at a 45-degree angle, then abruptly leveled off into a gravity-defying horizontal flight path. And, as LaPaz would note in a letter to the district commanding officer of the United States Air Force Office of Special Investigations, “none of the green fireballs has a train of sparks of a dust cloud…”

In the years since, there have been reports of green-fireball sightings around the world, from Alberta, Canada to South Africa. In June 2018, a green fireball made an impressive appearance at a concert performance in the Netherlands by the Foo Fighters (coincidentally, the band named for the U.S. pilots’ term for UFOs during World War II). And according to the International Meteor Organization there were more than 170 reported sightings of the fireballs that night, in at least five European countries. The band’s reaction, according to their official Twitter account: “The sky IS a neighborhood.”

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An alternate form of lightning?

The phenomenon came to the attention of the Australian physicist Dr. Stephen Hughes in 2006, when several green fireballs were spotted in the sky in Queensland and New Zealand. “I came to the conclusion that there was something a bit strange going on,” he says.

Hughes went on to write a paper that theorized a possible connection between green fireballs and the well-documented, but still ultimately little-understood, phenomenon of ball lightning—mysterious hovering orbs of electricity that have only been taken seriously by science since the 1960s, well after the New Mexico sightings.

There is still no conclusive theory of what ball lightning is, but hypotheses include antimatter, light bubbles, microwave interference, retinal after-images, electromagnetic knots, even primordial black holes.

Dr. Hughes’ own theory of ball lightning, which he believes fits the description of the New Mexico fireballs: electrified air. “It occurred to me, sometimes when something shoots through the atmosphere, like a meteor, it could be creating a conductive pathway from the ionosphere—a whole ocean of plasma above the Earth—down to the ground. The air becomes electrified.”

The phosphorescent green color, Dr. Hughes says, is due to ionized oxygen, which also accounts for the striking greens of the aurora borealis, also known as the Northern lights.

READ MORE: When UFOs Buzzed the White House and the Air Force Blamed the Weather

Cold War spy craft…or extraterrestrial probes?

The Los Alamos National Laboratory in New Mexico, where nuclear weapons were being designed for the Manhattan Project during WWII. Numerous green fireball sightings were reported in the vicinity of this and several other highly sensitive military and weapons-development sites. Jeffrey Markowitz/Getty Images

This potential explanation could not have occurred to those on the ground in New Mexico in 1948. After interviewing more than a hundred witnesses, Dr. LaPaz went on to advise the military and the Atomic Energy Commission of his opinion that the fireballs were likely either top-secret “unconventional defensive devices” being tested by the U.S.—or Soviet spying devices.

When Edward J. Ruppelt, director of the U.S. Air Force Project Blue Book UFO investigations, visited the Los Alamos National Laboratory in early 1952 to interview scientists and technicians, he noted that they became particularly animated when the idea of interplanetary vehicles was suggested.

“They had been doing a lot of thinking about this, they said, and they had a theory,” wrote Ruppelt in The Report on Unidentified Flying Objects (1953). They thought the fireballs were actually extraterrestrial probes “projected into our atmosphere from a ‘spaceship’ hovering several hundred miles above the Earth.”

Officially, government investigators concluded that the green fireballs were some kind of never-before-seen natural phenomenon. Interest in, and investigation into, the fireballs dropped off at the outbreak of the Korean War.

“Writing these off as natural phenomena did not solve the problem,” says UFO researcher Jan Aldrich, who believes the green fireballs were related to aerial phenomena spotted in Fort Hood, Texas, in 1949. “It just pushed it under the table.”

Nuclear fallout debris?

But that hasn’t stopped UFO researchers from speculating more recently.

In his 2008 book UFO and Nukes: Extraordinary Encounters at Nuclear Weapons Sites, Robert Hastings, drawing on declassified official documents, suggests that the fireball trajectories align with those of fallout-debris clouds associated with top-secret atomic testing.

But according to Dr. Hughes, there’s another reason to suspect those green fireballs were buoyant balls of plasma: All those unpredictable movements, which suggest their paths may have been following electric field lines above the Earth.

“Personally, I think that the erratic change in direction is reasonably conclusive proof that the phenomenon is electrical in nature,” says Dr. Hughes, citing the more familiar sharp angles of a lightning bolt streaking through the sky.

“If the ball lightning phenomenon was a solid mass, there would be enormous inertia, making it very difficult to explain the source of energy for such extreme acceleration. In the case of a plasma ball, an internal energy source is not required—in the same way that a bolt of lightning does not need some kind of electrical rocket motor to rapidly change direction on the way to the ground or between clouds.”

Still, at this stage, it’s hard to shake the sense that equating the green fireballs with ball lightning is tantamount to explaining a mystery with another mystery.

“I’m a believer in the sense I believe that UFOs exist,” says Dr. Hughes, who finds the name apt: “They are unidentified flying objects. I just don’t think there are little green men at the controls.”

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