Every year, thousands of people around the world claim to see them.

UFOs are the second-most reported paranormal phenomenon in the United States, after ghosts. In 2015, the world’s largest collaboration of volunteer UFO investigators, the Mutual UFO Network, or MUFON, documented more than 10,000 alleged sightings worldwide.

Many of MUFON’s self-titled field investigators have backgrounds in science and law enforcement, according to the Newport Beach, Calif.-based organization, which was founded in 1969. Its volunteers conduct in-person witness interviews, and sometimes document the sites of alleged UFO encounters.

Chase Kloetzke, a Brunswick resident and retired defense department contractor, has been probing UFO reports for MUFON since 1996.

“When you tell people you’re a UFO investigator, they conjure up mainstream media images of us running around chasing aliens in some Tennessee backwoods with some kids on moonshine or something,” she said with a laugh. “But that’s not at all what we do.”

When Kloetzke goes to gather a UFO report, she does not bring a tinfoil hat. Instead, she takes an array of supplies to take measurements, evidence samples and compiles real-world data. Her instruments can scan for radiation, electromagnetic fields and even heat signatures.

The closet in her home’s second-story bedroom looks a lot like a police department’s equipment locker. She has sealable evidence bags, yellow “do-not-cross” tape and even DNA testing kits.

Most of MUFON’s UFO reports make their way to local investigators after the alleged witness files a report on the group’s website. The MUFON member will contact the witness by phone and set up an in-person interview — usually at a public place, like a library of coffee shop.

“Initially, we would do an interview to try rule out things we can explain right away,” Kloetzke said. “We would have already done some background on the astronomy of the evening, because between 93 and 95 percent of our cases are explainable.”

Weather patterns, astronomical events like meteor showers, airplane flight paths — even swamp gas — are the types of natural anomalies Kloetzke and other investigators eye before jumping to conclusions, she said. Still, some cases aren’t that easy to explain.

One such case supposedly happened at about 7:20 p.m. October 30, 2015, when a Chamblee resident was driving with his father-in-law on Dunwoody’s North Shallowford Road, outside Atlanta. The two witnesses claimed to see “what appeared to be two parallel fireballs falling from the sky, moving together, their straight paths on a line angled down and left at about 20 (to) 25 degrees,” according MUFON’s case file, overviewed in its 2016 annual report.

Suddenly, the fireballs disappeared and, according to the witnesses’ statements, they saw a dark, triangle-shaped craft hovering stationary in the sky. Tree cover eventually blocked the witnesses’ view and the supposed craft disappeared, but a total of seven people claimed they saw the fireballs. The American Meteor Society also documented the event.

A detailed, 269-page manual published by MUFON lays out the procedure investigators are to follow when documenting such reports.

“It isn’t much different from a police-style interview,” Kloetzke said. “As investigators, we’re looking for evidence. We are looking for leads, and we’re looking for anything the witness can tell us that would take us in the direction of the truth.”

Kloetzke is not afraid of a witness being discredited by cold, hard facts. She wants the case file to be as thorough and scientifically based as possible. Detailed statements, photographs and sometimes maps plotting data points are logged and later published online for other investigators and the public to review. Kloetzke invites the scrutiny.

“If the public and critics can’t scrutinize the research, then what do we have?” she said. “That’s not only how science works, but also how we’ve learned to meet the burden of proof. We have to be dispassionate. We owe the witness the truth, and a scientific, dignified investigation, and that’s what we’re trained to do.”

Not everyone is so sure the process is ironclad. Sharon Hill, a licensed geologist and skeptic who runs the blog DoubtfulNews.com, thinks serious science may be best left to trained professionals. In her master’s thesis at the University of Buffalo, New York state’s flagship university, she argued volunteers and the public may not understand the intricacies involved in the scientific method.

“Science is a process,” she told The News by phone from her hometown of Harrisburg, Pa. “The process is not something we talk a lot about in culture. We tend to think about science in terms of the products it gives us. Most people don’t know a scientist, and a lot of people didn’t like science in school because they thought it was boring.”

Hill thinks the news media has a tendency to condense science into snappy headlines and oversimplify a complicated process.

Likewise, Hill said the proliferation of television shows like The History Channel’s “Hangar 1,” centered on UFOs, has normalized otherwise fringe theories. She said the reality-style shows may contribute to more people claiming paranormal experiences.

“If the media and culture are making that more accessible and less stigmatized, people are more free to explore ideas that were otherwise sort of off-limits,” Hill said.

While gathering research for her 2010 master’s thesis about science and society, Hill documented more than 1,000 self-described paranormal-research organizations in America. The vast majority were groups of so-called “ghost hunters.” Only two – MUFON and one other organization — were solely devoted to investigating UFOs.

The rise of “psuedoscience,” as Hill calls paranormal studies, concerns her. She worries the public may lose sight of what science has traditionally been.

“Science is about using rational, critical thinking,” she said. “It is one of the best ways for us to discover the world around us. We can ask ourselves, ‘What does science say?’ It’s about asking good questions and finding the best answers.”

But of 1,000 groups she examined, MUFON is among the most methodical and transparent, Hill said. MUFON, she notes, has chapters in every state, and a structured process that has been standardized, which helps to compare data.

“They have made an effort to be as careful as possible in documenting cases, and put it together in a way that people can access their data,” Hill said. “And that’s key to investigating those types of cases.”

Kloetzke, like Hill, thinks detailed data gathering is key to finding the truth. In fact, she wrote a manual about the process.

Published in 2014, Kloetzke’s book, “Admissible,” is more than 140 pages of instructions and advice on best practices for compiling information that is as unbiased as possible.

“It’s very important that we conduct a scientific investigation,” Kloetzke said. “Of course, we’re investigating things that don’t always leave a physical evidence, so we rely a lot on witness testimony.”

Still, when there is physical evidence, the manner in which it is gathered is important.

“We might be looking for radio signals, or things like that,” she said. “Mostly, it’s forensic evidence. A lot of reports talk about slag — molten metal — and we have several pieces of that. We follow a strict chain of custody for evidence, because we have to do it right, and we have to be under public and scientific scrutiny.”

Even as a skeptic, Hill agrees that any evidence — if gathered properly — can’t hurt the public’s understanding of strange phenomenon.

“I think it’s generally harmless,” Hill said of volunteer UFO investigating. “Of course, it’s a slippery slope. If we can find more data that shows a pattern, we might find something actually worth studying.”

Kloetzke remains undetered in her quest for proof.

“I absolutely believe that the UFO is a reality, there is no question” she said. “I think we don’t even know what we don’t know.”