Phoenix Lights: What caused one of the world's largest UFO sightings?

(Credit: Fox 10 Phoenix)

It has been 21 years to the day since a mass UFO sighting occurred in Phoenix, Arizona, with the mystery still unsolved.

On March 13, 1997, thousands of people reported a string of bright lights in a triangular formation flying in the sky like a UFO.

Dr. Lynne Kitei witnessed the event and said she still doesn’t know what she saw that day.

“It was a mile-wide formation of these orbs and I caught them head-on turning into a V,” she told 12 News.

The US air force identified the lights as flares dropped by an A-10 Warthog aircraft performing training exercises at the Barry Goldwater Range in southwest Arizona.

It’s a nice theory, but Dr Kitei doesn’t feel so sure it’s true.

“How can flares keep a formation, traverse the entire state and beyond for hours in a rock-solid V?” she said.

Another person who claims to have seen the lights was Hollywood actor Kurt Russell.

The Escape from LA star said he was traveling in a private plane into Phoenix when he spotted the phenomenon, quickly reporting it to the control tower at the airport.

“I’m going to declare it’s unidentified, it’s flying and it’s six objects,” he recounted during an appearance on The One Show.

Despite the many witnesses claiming the lights were from a UFO, aviation experts said the flare explanation does hold merit.

Former F-16 pilot Ty Groh said flares in the sky act like hot-air balloons and go where the breeze takes them, with a strong gust able to propel all of the flares at the same time at a uniform distance.

He added that extremely bright objects, like flares, can appear closer than they are in reality when dropped at a distance.

“You’ll be looking at airliners that look like they’re 10 miles (16km) away and they’re 400 miles (643) away,” he told 12 News.

Despite the air force’s explanation, the Phoenix Lights are still a mystery.

This story originally appeared in news.com.au.