On a January afternoon in 1981 a farmer in southern France heard a whistle in his garden and saw something settle on the terrace below him. Within a day gendarmes were on their knees collecting soil. Within two years a French government space agency had published the most detailed physical-trace report it would ever release. Forty-five years later, nobody agrees on what made the mark.

In late 1977, people on a small island at the mouth of the Amazon began arriving at the clinic with burns, puncture marks and stories of beams of light coming through their roofs at night. The Brazilian Air Force sent an officer to find out what was happening. Twenty years later, he broke his silence, and then he was dead.

On a September morning in 1994, the pupils of a small private school outside Harare came running in from morning break with the same impossible story. Thirty years, one Harvard psychiatrist and a shelf of crayon drawings later, nobody has explained what they saw.

On an autumn morning in 1966, a class of girls running laps on a Melbourne school oval stopped and pointed. Within minutes hundreds of students and teachers were watching a silver object sink behind the pines. By lunchtime they had been told to say nothing about it, and no official file has ever been produced.

On a clear March evening in 1997, thousands of people watched an enormous V of lights drift silently across 300 miles of Arizona sky. Two hours later a second set of lights hung over Phoenix and made the evening news. Nearly three decades on, the state's own governor is among those who say they still cannot explain what they saw.

At daybreak on 14 April 1561, the citizens of a great German city looked up and saw spheres, crosses and blood-red globes wheeling in combat above their rooftops. A single printed sheet preserved the account. Nearly five centuries later, we still argue about what they saw.

It came from another star, tumbled like nothing we had ever seen, and accelerated without a trace of exhaust. Scientists still argue about what slipped through our solar system in 2017.

On an October night in 1967, a fishing village watched lights fall into the sea. Police, the Coast Guard and Navy divers went looking. The file is still marked unsolved.

In 2021 the U.S. government stopped laughing off strange objects in the sky and started counting them. What followed was not proof of aliens, but something rarer: an official paper trail of honest uncertainty.

In 1989 and 1990 thousands of Belgians, many of them police officers, reported a vast silent triangle. The Air Force scrambled fighters, published its radar data — and never identified what it chased.

In September 1976, two Iranian F-4 Phantoms closed on a brilliant light over Tehran — and their instruments and weapons died on approach. A US intelligence file calls the report outstanding. Nothing calls it solved.

In 1986 a veteran 747 captain reported an object he said dwarfed his jumbo jet, pacing him for 400 miles across the Alaskan twilight. The FAA logged the crew statements, the radar and the voice tapes - and to this day cannot confirm a thing.