The television show Unsolved Mysteries debuted in January of 1987 and featured peculiar cold cases about things like missing persons, murders, and paranormal activity. It’s been extremely popular, and thanks to its wide and fervent viewership, many of the cold cases have actually been solved. They’ve even exonerated a few wrongfully-convicted inmates along the way.

That said, despite the attention, a number of the 1000 cases remain open. Below are 7 of the 10 that co-creators John Cosgrove and Terry Dunn Meurer told Mental Floss really stuck with them.

#1. The Kecksburg U.F.O. Incident

On December 9, 1965, thousands of people reported a strange light over the northeastern United States and Canada. Citizens of Kecksburg, PA, saw it too, but also saw a commotion around what seemed to be a crash site. There were reports of the crash being a meteor or space debris, but UFO researchers have insisted an extraterrestrial craft was to blame.

NASA and the Air Force have to this day declined to identify what landed in Kecksburg that night.

#2. A.W.O.L.

This case followed Private Justin Burgwinkel, an Army man who aspired to become a Ranger and was committed to a career in the military. Then out of nowhere, he began to act oddly at home, abruptly leaving his girlfriend for meetings and telling her they were secret. After three years, Burgwinkel vanished. His car was found at a motel filled with all of his personal effects, and his dog tags were among them. He had once told his girlfriend that if they were found, “that means I’m dead.”

No one has seen or heard from him since June 12, 1993.

#3. One Minute Million

On April 19, 1989, an armored car in Eden Prairie, MN, was stopped by a gang of armed robbers that got away with a million dollars in under a minute. It was the second of what would be three similar heists within a couple of years, but the case was never solved.

Police suspect that, because of the precise nature of the attacks, the gang might have been ex military.

#4. Dial H for Abduction

In 1991, Angela Hammond was talking to her boyfriend Rob on a pay phone and commented that a green Ford pickup had been circling the block, and that a “filthy, bearded man” had exited and was using the phone next to hers. They were still on the phone when Hammond screamed and stopped responding to Rob.

Rob hopped in his car and drove to the phones, passing the kidnappers in a pickup truck with Hammond inside screaming his name on the way there. He lost them when his transmission failed. The only clue he had to go on was the truck had a giant decal on the back window.

The boyfriend was originally a suspect, but was soon cleared, and there have been no leads since. Police do think that Hammond’s disappearance could be related to other abductions near Clinton, MO, where she lived.

#5. D.B. Cooper

D.B. Cooper is an alias for a man or woman who successfully hijacked a plane bound for Seattle in November of 1971. Cooper told a stewardess he had a bomb and asked for $200k when the plane landed. When he got his money, he jumped out of the plane with a parachute. The police have never come up with a single suspect.

In 2018, an amateur sleuth named Rick Sherwood suggested that Cooper might be Robert Rackstraw, a Vietnam vet with parachuting experience, but the FBI hasn’t made any further comments.

#6. Dreamy Disappearance

Cynthia Anderson began having a recurring nightmare in which she let someone into her house who meant her harm. Around the same time, she began receiving threatening calls at work, to the point where her employers installed an emergency buzzer at her desk.

Then, in August of 1981, when the attorneys she worked for arrived at the office, she wasn’t behind her desk. The police later received two phone calls stating that Anderson was being held in a basement of an occupied house. However, without any more information to go on, the case has never been solved.

#7. Tupac Shakur

The puzzling details that led to the 1996 murder of rapper Tupac Shakur have fascinated the public since they came to light on this show and elsewhere. Tupac was in Vegas to watch a Mike Tyson boxing match. After the fight, it was rumored that he and fellow rapper Marion “Suge” Knight got into an altercation with the Crips — a notorious street gang — while they were at a stoplight, and a white Cadillac pulled up and opened fire.

Knight was injured and Shakur died from his wounds. A witness was later gunned down at his home in New Jersey. No one has ever been arrested for either crime.