There's no denying that America has an enduring fascination with unidentified flying objects, or UFOs. However, UFO interest extends far beyond the U.S. — sightings are reported worldwide, and multiple observations in far-flung locations describe aerial objects that are uncannily similar to each other, Luis Elizondo, former head of a top-secret U.S. government agency tasked with investigating UFOs, recently told Live Science.

Though some label UFOs as alien spacecraft, the term simply describes aerial objects that defy explanation. One possibility is that they represent technology deployed by a hostile human source, so it's impossible to say for sure that UFOs are harmless, Elizondo said. Evaluating the potential threats posed by UFOs should therefore involve the collaboration of leaders around the world, said Elizondo, who left the Pentagon in 2017 and is now a director of global security and special programs at To the Stars Academy of Arts and Science, a private agency pursuing evidence of UFOs.

"I think we're at the point now where we're beyond reasonable doubt that these things exist," Elizondo said. "We know they're there — we have some of the greatest technology in the world that has confirmed their existence." But where do these objects come from, what are their capabilities and what are the intentions of whoever may have sent them? Elizondo and other experts delve into these questions in the second season of the series "Unidentified: Inside America's UFO Investigation," with the first episode airing tonight (July 11) on the History Channel.

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In the show's new season, Elizondo and Chris Mellon, a former Deputy Assistant Secretary of Defense and Intelligence, piece together eyewitness accounts and other clues about intriguing, unexplained sightings by military personnel and civilians, according to the series website.

UFOs are also known as unidentified aerial phenomena, or UAPs, and the U.S. government has been collecting reports of these enigmatic objects since the 1950s: in the Air Force's Project Blue Book, from 1952 to 1969, and through the National Investigations Committee on Aerial Phenomena (NICAP), a federal agency that compiled witness accounts of UFO encounters from the 1950s through the 1980s.

Project Blue Book investigated more than 12,000 reports of UFO sightings. Most of those turned out to be misidentification of aircraft, weather balloons, clouds or starlight, but 700 incidents were left unresolved.

The UFO encounter in the "GOFAST" video clip was recorded in 2015 by a U.S. Navy pilot flying a F/A-18 Super Hornet during military maneuvers off the eastern coast of the United States. (Image credit: U.S. Department of Defense)Long-standing stigma and government secrecy surrounding UFOs have encouraged people to dismiss sightings as hoaxes or jokes. But as long as the origins and capabilities of even a few of these objects remain unknown, it would be foolish to not take them seriously, Elizondo explained.

"There's something in our sky and we don't know what it is, we don't know where it's from. Is that a problem? From a national security perspective, yes, it's a problem," he said. "We need to understand what these are, in order to make a determination if they're a threat."

Elizondo, a former military intelligence officer, led the Pentagon's Advanced Aerospace Threat Identification Program (AATIP), which formed in 2007 to probe reports of unexplained aerial sightings and reportedly shuttered in 2012, Live Science previously reported.

As with Project Blue Book, a number of AATIP's UFO cases turned out to be misidentifications or technology malfunctions — but some UFOs remained unidentified. Over time, Elizondo's involvement with AATIP led him to the realization that the bureaucracy of the system was failing the public, keeping information about UFOs secret and downplaying the risks they might pose.

"That's really what led me to resign," he told Live Science.

No elegant solutions

Many of the UFO sightings that AATIP investigated were recorded by members of the military in restricted airspace. Among them were three mid-air encounters that U.S. Navy pilots captured on video in 2004 and in 2015; the footage was officially declassified and released online on April 27. Other instances involved UAPs flying at what appeared to be hypersonic speeds — more than five times the speed of sound.

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None of the objects had visible wings or other means of propulsion. What's more, they appeared to be performing maneuvers that would have subjected them to as much as 700 times the normal pull of gravity, or 700 Gs, Elizondo said. (Of course, there is no way to confirm those estimates, as the sightings were so fleeting and much of the obvious documentation is not readily available.) To put that into perspective, airplane cockpits can withstand only about 18 Gs before cracking, and people can typically endure just a few seconds at 9 Gs before losing consciousness, as gravity draws blood into the extremities and oxygen ceases to flow to the brain, according to PBS.

"It would be my hope that we can find elegant solutions to what these things are," he said. "If you can show me one technology that mankind has ever been able to build that does that, great! But so far no one's been able to show that, to me or anybody in the U.S. government."

The new season of "Unidentified: Inside America's UFO Investigation" begins July 11 on the History Channel at 10 p.m./9 p.m. CT.

Originally published on Live Science.