There is nothing worse than losing your child, which is why parents usually keep their little ones at arm’s length or with a trusted babysitter. On the night of May 3, 2007, Madeleine McCann’s parents did neither — they left her sleeping in their rented apartment in Portugal and went out for tapas with some friends.

Now, lots of people tsk-tsk that decision right off, but keep in mind that the McCanns were literally across the street — according to the Telegraph, the tapas bar was just 50 yards away from the apartment. Still, it was too far away for them to notice what happened that night. Somehow, Madeleine disappeared and no one knows how or who might have been responsible.

Worse, the McCanns were initially treated as suspects by the Portuguese police, who were so sure that Madeleine’s parents were responsible for their daughter’s disappearance that they bungled the early investigation — around 20 people went in and out of the apartment before it was sealed off as a crime scene, and there were no roadblocks put into place until long after whoever took Madeleine would have been out of the area. So it’s probable that the police missed important leads and that evidence was contaminated or destroyed, thus making the task of finding the missing girl a lot more difficult.

There have been many theories but very little evidence about what actually happened to Madeleine McCann. The only thing police know for sure is that she’s still missing.

Lars Mittank was a pretty typical German tourist traveling in Bulgaria with some friends, until he got into a strange altercation with four men in a pub. According to Ranker, Mittank suffered a ruptured eardrum in the fight and was advised to stay in Bulgaria a little longer while the injury healed.

Here’s where the story gets weird. The doctor who saw Mittank prescribed cefuroxime, an antibiotic. After that, Mittank became paranoid — he reportedly called his mother and told her that there were men following him and that they wanted his medication. Concerned, his mother booked him a flight home, but he never got on the plane. Mittank made it to the airport and visited medical services, but the doctor who saw him said he become anxious and declared “I don’t want to die here” before sprinting out of the terminal without his luggage. Witnesses say he ran across the parking lot and climbed a fence. He was never seen again.

So what happened to Lars Mittank? Medical journals have noted cases of psychotic reaction to the antibiotic cefuroxime. His extreme paranoia could have also been compounded by a head injury sustained in the fight. Or maybe someone really was following him. Either way, it’s a mystery what happened to him after he scaled that fence and disappeared.

Sixty-one-year-old Mel Nadel was on a hunting trip in the Santa Fe National Forest in September 2009 when he vanished. At around 4:30 on the day he disappeared, he told friends that he was just planning to venture out a short distance and would be back before nightfall. (Although conflicting sources say he stayed behind at the campsite while his friends went out to hunt and was gone when they returned.)

According to the Charley Project, Nadel left most of his gear including his GPS unit inside his locked Jeep Cherokee and carried only a revolver and a hunting bow with arrows. The prevailing theory is that he became lost or injured, though his family doesn’t think that adds up. “I feel like this is criminal,” his daughter said three weeks into the search. “… I honestly don’t think he just got lost because we would have found him by now.”

And it’s not like the guy was especially frail — he was a fitness instructor who owned a pilates studio, and he was also a first-degree black belt in tae kwon do. It gets even weirder. Shortly after Nadel’s disappearance, dogs followed his scent into the woods for about 150 yards, and then the trail suddenly just ended, as if he’d vanished in place. Three years later, his wife said the family was still looking for answers. “Even a trace of his belongings would help,” she said. “We are in limbo.”

Yosemite is a beautiful place, but as far as national parks are concerned, it does seem to have some sociopathic tendencies. According to National Parks Traveler, there are “at least” 60 unresolved cases of missing travelers in the American national parks system as a whole — and Yosemite alone has had 30 plus missing persons cases since 1909, 12 of which are considered cold or unresolved with no active leads. One of these is 14-year-old Stacey Arras, who was on a mule pack trip in 1981 with her father and eight other people when she wandered off to take photographs and never came back.

More than 100 people, including 50 volunteers, six dog teams, professional search and rescue teams, divers and mountain climbers scoured the area in search of the missing teen. A helicopter spent 40 hours in the air searching for some sign of Arras, but no trace of her was ever found. Officials said she could have fallen into a number of different places where searchers would be unable to find her, and it was also possible she might have hiked to the road in search of help. Or she could have been abducted — given that nothing but her camera’s lens cap was ever recovered, that does seem like the most likely scenario. The lens cap wasn’t a useful clue, though, and the search was called off 11 days after Arras’ disappearance. She was never seen again.

For a lot of people, solo travel is cathartic. But people who travel alone are especially vulnerable to just about every travel-related danger there is, from ill-intentioned strangers to accidents to getting lost in the woods. So if you’ve learned nothing from all of these stories, learn this: If you must travel alone … just don’t. Don’t travel alone. It’s a terrible idea.

According to the Charley Project, Karen Denise Wells was traveling alone when she vanished in April 1994. She was 23 years old and planning to visit a childhood friend in Bergen, New Jersey. On April 12, she stopped at a motel and called her friend to say she was planning to pick up dinner at McDonald’s and then go to bed.

Just after midnight, Wells’ friend arrived at the motel and tried to wake her, but Wells wasn’t there and her bed hadn’t been slept in. The next morning, her rented car was found damaged and covered in mud in the westbound lane of Route 274 with both the passenger and driver doors wide open. The battery was dead, the car was out of gas, and there were uneaten french fries, empty soda bottles, and a pair of shoes inside. Weirder still, there was an extra 700 miles on the car that couldn’t be accounted for based on Wells’ travel plans.

So what happened to Karen Denise Wells? Decades later, we still don’t know.