Within the larger mystery of the UFO phenomenon is another, still-unsolved puzzle: Why do so many reports involve strange, triangular-shaped craft—often described as dark in color, virtually noiseless and the size of a football field or larger? What, exactly, are they? And why are so many witnessed hovering or moving slowly and methodically, with no visible contrails?

In the years after the U.S. Air Force coined the term “unidentified flying object” in 1952, reports often referred to UFOs generically as flying saucers. But witnesses then, and since, have described a wide array of shapes: saucers (or two saucers put together), eggs, hats, cigars, boomerangs, lightbulbs—even Tic Tac candies.

Among the most commonly reported shapes were V-shaped, arrowhead-like or triangular. David Marler, UFO researcher and author of Triangular UFOs: An Estimate of the Situation, says he has reviewed more than 17,000 case files involving unidentified triangular craft, sometimes called “black triangles.” Whether the sightings represent advanced U.S. spy craft—as some speculate—or something of unknown origin, their purpose remains mysterious. Given their consistent hovering behavior, Marler says, they might be engaged in “surveillance of some nature—or scanning. Or analyzing the topography.”

“There have been many instances in which these vehicles have been observed over bases operated by the Strategic Air Command,” says Chris Mellon, former deputy assistant secretary of defense for intelligence during the Clinton and George W. Bush administrations, whose career has focused on unconventional threats to American security. Mellon is now an integral part of the investigative team featured on HISTORY's “Unidentified: Inside America’s UFO Investigation.”

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In the 1950s, ‘60s and ‘70s, triangular UFO reports hailed from across the U.S. and beyond. During the 1960s, at the height of Cold War UFO fever, mysterious flying triangles were reported over Connecticut, Georgia, Pennsylvania and Texas—as well as London, Madrid and Czechoslovakia. In 1969, two National Guard pilots tailed a “triangular shaped object, 50 feet in diameter” for 20 minutes over San Juan, Puerto Rico, until they ran low on fuel and had to return to their base. Many of these incidents would be attributed by officials to atmospheric conditions, weather balloons or other everyday sources, but some remained unexplained.

Between 1983 and 1986, a notable rash of mass sightings occurred in New York’s Hudson Valley, some 50 miles north of New York City. One witness, Kevin Soravilla, a retired lieutenant from the Yorktown Police Department, described a huge, silent craft, 100 yards from wingtip to wingtip, hovering low, which banked and made a 45-degree turn before abruptly speeding off. Soravilla said he called Stewart Air Force Base in nearby Newburgh to determine whether one of its C-5 transport planes—then the world’s largest and heaviest aircraft—had been in the skies that night; none had. Later that year, a hulking triangular UFO hovering over a stretch of New York’s Taconic Parkway prompted a huge traffic pile-up as scores of motorists stopped to get a better look. Similar incidents continued in the region for several years.

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Many witnesses describe what they perceive to be the crafts’ extraordinary abilities. One evening in late November 1989, two police officers on patrol in Eupen, Belgium, not far from the German border, spotted an odd triangular object overhead. In the ensuing days, hundreds of Belgians reported similar UFOs, described in news reports as “a triangular object with a bright red center light” or as a “flying platform” with three huge searchlights.

In March 1990, the Belgian air force sent up two F-16 fighter jets to get a closer look at one triangle that had been spotted on radar. Their onboard computers recorded the object’s remarkable maneuverability and its ability to accelerate from 1,000 kilometers per hour (about 621 miles per hour) to 1,800 kilometers per hour (about 1,120 miles per hour) within seconds. “What the computers registered exceeded the limits of conventional aviation,” a Belgian air force colonel told reporters.

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In March 1997, Phoenix, Arizona, became a UFO hotspot when some 30,000 local residents saw something strange in the skies. Some reports said the mysterious object was V-shaped, but many described it as triangular. “It was in a triangle shape and it had three lights. It was moving very slowly,” an 11-year-old Cub Scout was quoted as saying. A retired airline pilot described it as “the size of 25 airliners…and it didn’t make a sound.” Others described it as the size of three football fields.

In 2000, police officers from neighboring municipalities in southern Illinois were called to investigate a trucker’s report of a massive arrowhead-shaped craft hovering low in the sky, two stories high and as long as a football field. Dispatch tapes reveal the shock and awe expressed by the different law-enforcement teams, who were all in radio contact with each other.

The National UFO Reporting Center, which catalogs more than 8,100 sightings of triangle-shaped UFOs since the early 1960s, lists more than 200 in the first half of 2020.

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Many of these sightings have been investigated repeatedly by UFO sleuths. The Belgian triangles have been explained away as stars, planets, balloons or blimps, with a bit of mass hallucination thrown in. The lights over Phoenix were dismissed as flares dropped during an Air National Guard exercise, although that theory has many skeptics. Some say the New York sightings were a hoax perpetrated by local stunt pilots flying in formation.

One explanation raises the possibility of the “airship effect.” That’s the theory that people who see unrelated lights in the sky can trick themselves into believing they are all part of the same object. Three lights? Must be a triangular spaceship. Three lights hundreds of yards apart? Must be a really big triangular spaceship.

Other speculation has focused on top-secret aircraft. Although the U.S. government has largely stayed mum on the matter, it’s common knowledge that the Air Force has experimented with triangular- and V-shaped aircraft for decades, including the B-2 Spirit stealth bomber and the F-117 Nighthawk—and possibly others kept under wraps. Sightings near the clandestine spy plane test facility at Area 51 in Nevada may indeed be connected to test flights of some of these craft.

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However, the extraordinary size many witness describe is puzzling. And for Marler and others, the volume of the sightings and the consistency of the crafts’ hovering behavior, combined with their unexplainable sudden accelerations, point away from known military technology.

If not home-grown, then what? One theory suggests that these craft are engaged in mapping sensitive sites. The southern Illinois sightings occurred within one to two miles of Scott Air Force Base, home to U.S. Air Mobility Command, which coordinates all global transportation for American troops. The Hudson Valley sightings happened in close proximity to Stewart Air Force Base. And Mellon has interviewed multiple Persian Gulf veterans who witnessed triangular craft near sensitive military operations. “An adversary planning a future attack would want to know every inch of the battlefield,” he says.

Still, the black triangle mystery persists. “There’s a lot of data,” says Marler. “That doesn’t equate to answers.”

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