On a humid, August night in 1952, scoutmaster D.S. “Sonny” DesVergers emerged burned and barely coherent from a dense palmetto grove in the South Florida Everglades. He claimed he had encountered an unidentified flying object that discharged a fireball, which left him singed and barely able to see.

Captain Edward J. Ruppelt, chief UFO investigator for the U.S. Air Force, would later label the event “the best hoax in UFO history.” But the DesVergers incident remains one of the most intriguing cases from Project Blue Book, the Air Force’s now-declassified investigations into UFOs—because it wasn’t just a sighting incident, but one involving a purported attack. To this day, it’s still unsolved.

Cue appropriately spooky "X-Files" music.

A series of investigations conducted by the U.S. Air Force between 1952 and 1969, Project Blue Book was tasked with scientifically analyzing UFO-related incidents to determine whether they were a threat to national security. Some say the project was commissioned to find rational explanations for these mysterious phenomena, to help quell a growing Cold War-era public hysteria over unidentified objects in the sky. UFO fever reached such intensity that in April 1952, four months before the DesVergers incident, LIFE magazine published a story called “Have We Visitors from Space?”

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

Pulling over to inspect a bright flash of light

As Ruppelt would later chronicle in his 1956 book The Report on Unidentified Flying Objects, on the evening of August 19, 1952, hardware-store clerk and Scoutmaster DesVergers, 30, was driving a group of Boy Scouts home when he saw a bright light flash over Military Trail near West Palm Beach, Florida. Thinking it may be a downed plane or car accident, DesVergers pulled onto the shoulder of the highway so he could take a closer look. Armed with a machete and flashlights, he entered the palmetto grove near where he saw the lights, leaving the three boys in the vehicle with instructions to alert the residents of a nearby farmhouse if he did not return in 15 minutes.

According to the declassified documents, after about four minutes of hacking through the bush DesVergers entered a clearing in the grove. The first thing he described was an acute, nauseating smell and then the feeling of somebody or something watching him. He next experienced a sensation of oven-like heat coming from above. Looking up, DesVergers said, he could not see any stars as he was standing beneath a hovering object.

The object was circular, DesVergers recounted, dull black, with no seams, about 30 feet in diameter with a height of 10 feet, a convex dome atop it and the bottom edge lit with a phosphorescent glow.

READ MORE: The First Alien-Abduction Account Described a Medical Exam with a Crude Pregnancy Test

Enveloped by a red mist

What happened next is what separates DesVergers’ encounter from thousands of other UFO sightings: As he slowly moved backward, he recalled, he heard a noise like metal against metal, “like a hatch opening,” after which a red, flare-like light came from the side of the object and slowly moved toward him. (DesVergers constantly referred to it as a “ship” when recounting the tale to the authorities) As he placed his hands over his face—fists closed, hand over each eye—the red ball of light grew into a red mist, engulfing him. It was then, he recounted, that he lost consciousness.

When he awoke, DesVergers said, he was leaning against a tree, but could not see properly as his eyes burned. Scrambling back through the palmettos, his eyesight slowly returning to normal, he burst, incoherent, out onto the highway, where he was met by the boys and local authorities.

‘I’ve never seen anyone as terrified as he was’

The three scouts, Bobby Ruffing, 12, David Rowan, 11, and Chuck Stevens, 10, remained in the car after DesVergers entered the grove. Later, in recounting what he witnessed to authorities, Ruffing said he initially saw a semi-circle of white lights descending into the trees. Ruffing also recounted seeing a red light through the brush, as did Rowan and Stevens, who told of also seeing DesVergers’ flashlight through the trees before going dark. That’s when the scouts headed to the nearby farmhouse for help; a Palm Beach County deputy and Lake Worth constable responded to the farmer’s call for assistance.

Returning to the site of the abandoned vehicle almost an hour after DesVergers first said he saw the lights, the officers and scouts witnessed the scoutmaster emerge from the palmettos, waving his machete and babbling incoherently. “In all my 19 years of law-enforcement work, I’ve never seen anyone as terrified as he was,” the deputy is recorded as saying in Ruppelt’s investigation.

Sketches by DesVergers in the Project Blue Book file further explaining his encounter. The Project Blue Book Archive/The United States Air Force

READ MORE: Meet J. Allen Hynek, the Astronomer Who First Classified UFO 'Close Encounters.'

Back at the Palm Beach County Sheriff’s office, DesVergers and the boys underwent questioning. Officers noted that the hair on DesVergers’ forearms was singed and the skin burned. They also noted three tiny burn holes in the bill of the scoutmaster’s cap.

Following procedure, the local authorities contacted relevant agencies with the incident report, which eventually made its way to Blue Book chief Ruppelt. He later described the case as “one of the weirdest UFO reports that I came up against.”

The mystery of the singed grass

Arriving in Florida soon after the encounter, Ruppelt and his team began their investigation, obtaining statements from all parties involved and taking grass and soil specimens from the clearing in which DesVergeres said the encounter took place. The latter evidence would prove to be the most inexplicable piece of the encounter puzzle.

“The fact that they documented and took samples at all is lucky, and one of the most interesting aspects of this case,” says Jeffrey Wilson, a private-industry analyst who examines noteworthy ground phenomenon. As co-founder of the Independent Crop Circle Researchers' Association (ICCRA), Wilson investigates global circle phenomena. Though different to the crop circles he examines today, aspects of the DesVergers incident led him to further investigate.

Grass samples taken from the site of the incident. The Project Blue Book Archive/The United States Air Force

As the grass specimens were being tested, DesVergers’ character would come under intense scrutiny, with authorities noting his other-than-honorable discharge from the U.S. Marines due to theft of a car, and what Florida locals would describe as his ability to tell tall tales. But when Ruppelt first interviewed DesVergers, he described the scoutmaster as likable, willing to cooperate and displaying the “immediate impression he was telling the truth.”

Taking into account the background checks on DesVergers, along with a return visit to the encounter site where he determined the Boy Scouts could not have witnessed DesVergers and the mysterious red light in the grove due to their distance and denseness of the foliage, Ruppelt would later call the entire event a hoax. DesVergers was painted as an opportunist and media-hungry conman who sold his story to The American Weekly newspaper the following year.

READ MORE: The UFO Sightings that Launched ‘Men in Black’ Mythology

Though Ruppelt would come to believe the tale was fabricated, and he and his team would come up with dozens of ways the event could have been staged, they never managed to prove the incident was, in fact, a hoax. Their biggest stumbling block: the grass samples taken at the site.

After samples from the Florida clearing were sent to Battelle Memorial Institute (under contract with the USAF to provide scientific support to Project Blue Book), agronomists made some interesting findings: Though the soil remained consistent, the root structure of the plants in question were charred black, and the lower leaves had deteriorated as if by heat. The only way the lab could come close to duplicating the effect was to place live clumps of grass in a pan of sandy soil and heat it to about 300 degrees Fahrenheit.

Though Wilson has witnessed singed grass in his investigations into ground phenomenon, it has always been an occurrence above the soil, never the roots, as the lab findings in the DesVergers case indicate. Wilson says this is the only recorded example of such findings of which he is aware.

With those associated with the case no longer able to comment or add context (DesVergers and Ruppelt have both since died), the case remains unexplained. But according to Wilson, “Something unusual happened to the guy, and the physical evidence backed him up. That’s why I put the effort into checking this out.”

“Why would you go to the trouble of faking something like this?” he continued. “Why, and how, would he stage that? It doesn’t make any sense.”

WATCH: Full episodes of Project Blue Book online now.