August 25, 1951 was a quiet summer night in Lubbock, Texas. That evening, a handful of scientists from Texas Technical College were hanging out in the backyard of geology professor Dr. W.I. Robinson, drinking tea and chatting about micrometeorites. It was quite the brain trust: chemical engineering professor Dr. A. G. Oberg, physics professor Dr. George and Dr. W. L. Ducker, head of the petroleum-engineering department.

Which made the story of what they witnessed that night all the more curious.

“If a group had been hand-picked to observe a UFO, we couldn’t have picked a more technically qualified group of people,” wrote U.S. Air Force Captain Edward J. Ruppelt later in his definitive 1956 casebook, The Report on Unidentified Flying Objects.In the early 1950s Ruppelt served as lead investigator for Project Blue Book, the official Air Force investigations into UFO sightings, after working on its precursor effort, Project Grudge.

READ MORE: Interactive Map: UFO Sightings Taken Seriously by the U.S. Government

Sightings of the blue-green lights kept growing

Around 9:20 p.m., the university colleagues saw something otherworldly in the expansive Texas sky: a V-shaped formation of 15 to 30 blueish-green lights passing overhead. Stunned, but still using their trained scientific reasoning, they figured the lights would reappear. And they did, about an hour later, in a more haphazard formation. The scientists were all in agreement: They had witnessed something fantastic—but what was it?

The professors weren’t the only credible witnesses to the mysterious blue-green lights that night. At dusk, in Albuquerque, New Mexico (about 350 miles away from Lubbock), an employee of the Atomic Energy Commission’s top-secret Sandia Corporation—a man with a high-level “Q” security clearance—had been sitting outside with his wife. According to Ruppelt:

They were gazing at the night sky, commenting on how beautiful it was when both of them were startled at the sight of a huge airplane flying swiftly and silently over their home… On the aft edge of the wings, there were six to eight pairs of soft, glowing, bluish lights.

Edward Ruppelt oversaw Project Blue Book for the U.S. Air Force, a program that monitored and investigated UFO reports. Everett Collection

An hour or so after, according to a retired rancher from Lubbock, his wife had seen something terrifying in the night sky. Ruppelt described it this way:

Just after dark, his wife had gone outdoors to take some sheets off the clothesline. He was inside the house reading the paper. Suddenly his wife had rushed into the house…“as white as the sheets she was carrying.” The reason his wife was so upset was that she had seen a large object glide swiftly and silently over the house. She said it looked like “an airplane without a body.” On the back edge of the wing were pairs of glowing bluish lights.

By the time Ruppelt flew into Lubbock to investigate the sightings in late September, hundreds of residents had seen the lights over a period of two weeks.

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Locals investigate, and even snap some photos

But not everyone had waited for the government to start looking into the matter. After alerting local papers like the Lubbock Avalanche-Journal, the Texas Tech professors started their own informal investigation. In the weeks after their initial August 25sighting, they and their friends observed the lights 12 more times. They measured the lights’ angles, roughly calculated their speed and noted that they always traveled from north to south. Armed with walkie-talkies, the scientist-sleuths and their friends formed two teams and attempted to measure the UFO’s altitude, with little success.

As the days went on, more and more Lubbock residents claimed to have seen the lights. And when the professors cross-checked these reports against what they themselves had seen and recorded, many of the facts lined up, Ruppelt wrote. Of course, few if any had recorded the phenomena with the same level of detail as the professors.

But while many observers offered incomplete or poorly expressed recollections, there’s little doubt that whatever people were seeing was something real. UFO sightings are usually one-off events, but these blue-green lights were observed multiple times, by hundreds of people.

READ MORE: Why Mysterious Green Fireballs over New Mexico Worried the U.S. Government in 1948.

The Lubbock Lights, photographed by 19-year old Carl Hart, Jr. on August 30, 1951 in Lubbock, Texas. Bettmann Archive/Getty Images

Plus, for many, there was physical proof: black-and-white photos taken by a Texas Tech freshman named Carl Hart, Jr. On August 31—the same night an Air Force wife and her daughter claimed to have seen a UFO while driving northwest from Matador, Texas, to Lubbock—Hart was keeping vigil in his bedroom, looking out for the infamous lights. According to Ruppelt:

It was a warm night and his bed was pushed over next to an open window. He was looking out at the clear night sky, and had been in bed about a half hour, when he saw a formation of the lights appear in the north… cross an open patch of sky, and disappear over his house. Knowing that the lights might reappear as they had done in the past, he grabbed his loaded Kodak 35, set the lens and shutter at f 3.5 and one-tenth of a second, and went out into the middle of the backyard. Before long, his vigil was rewarded when the lights made a second pass. He got two pictures. A third formation went over a few minutes later, and he got three more pictures.

These hotly debated images, which show a cluster of dim lights in a V-formation moving through the night sky, are the only visual representation of what hundreds were now claiming they saw.

READ MORE: Mysterious UFOs Seen by WWII Airmen Still Unexplained.

Captain Edward Ruppelt, standing between the two seated men, with other officers of the U.S. Air Force at a 1952 news conference where they announced the installment of more than 200 cameras in attempts to obtain data on the unidentified flying objects reported from various parts of the nation. Bettmann Archive/Getty Images)

Was it birds? Or planes? The government's investigator goes coy

As Ruppelt began his formal investigation, he found that the lights had affected all who saw them, including a hardened old man from Lamesa, who had witnessed them with his wife. “He broke off his story of the lights and launched into his background as a native Texan, with range wars, Indians and stagecoaches under his belt,” Ruppelt recalled of their interview session. “What he was trying to point out was that despite the range wars, Indians and stagecoaches, he had been scared. His wife had been scared, too.”

The old Lamesa man had suggested that the lights were actually plover birds, a theory to which Ruppelt would lend some credence. But just like many people Ruppelt interviewed, the old man admitted he and his wife had been looking for the lights after reading about them in the paper. This was a common thread tying together many of the witnesses. “One point of interest was that very few claimed to have seen the lights before reading the professors’ story in the paper,” Ruppelt wrote. “But this could get back to the old question, ‘Do people look up if they have no reason to do so?’”

READ MORE: Meet J. Allen Hynek, the astronomer who first classified UFO 'close encounters.'

So, what exactly did all these people witness? In The Report on Unidentified Flying Objects, Ruppelt—by all accounts an honorable and fair man who oversaw what many describe as the “golden age” of the government’s official UFO investigations—offers a strangely evasive explanation:

I thought that the professors’ lights might have been some kind of birds reflecting the light from mercury-vapor street lights, but I was wrong. They weren’t birds, they weren’t refracted light, but they weren’t spaceships. The lights that the professors saw…have been positively identified as a very commonplace and easily explainable natural phenomenon…I can’t divulge exactly the way the answer was found because it is an interesting story of how a scientist set up complete instrumentation to track down the lights. Telling the story would lead to his identity and, in exchange for his story, I promised the man complete anonymity... With the most important phase of the Lubbock Lights “solved”—the sightings by the professors—the other phases become only good UFO reports.

And so, the mystery of the Lubbock Lights remains unsolved.

“The Lubbock Lights incident persists in the memory of many older citizens, and to this day captivates researchers from across the country,” Dr. Monte L. Monroe, Southwest collection archivist at Texas Tech University told Texas Highways Magazine. “Mention the event, and everyone has an opinion. Some believe the bright, semicircular, so-called ‘string of beads’ crossed the sky at great speed, high in the stratosphere. Few agree with the streetlight-illuminated, migratory duck-bellies theory ventured at the time by skeptics or in the Air Force report.”

According to Monroe, the professors and other witnesses—tired of explaining themselves and what they saw—almost totally ceased giving interviews by the 1970s. In a rare informal interview, more than 40 years after the sightings, Carl Hart, Jr. reportedly told author and UFO researcher Kevin D. Randle he still had no idea what he had photographed that pleasant August night many moons ago. But like hundreds of others witnesses in and around Lubbock that strange Texas summer, he saw something he would never forget.

WATCH: Full episodes of Project Blue Book online now.