The Greatest Unsolved Mysteries Of All Time Will Keep You Fascinated For Days

On most television shows, mysteries tend to have a short lifespan. They might last for a season or two, or maybe even the length of a series, but in the end, writers know that audiences want closure. For some reason, though, people are fascinated with unsolved mysteries in real life. On TV, an unsolved mystery is unsatisfying (or means that the show got cancelled). In the real world, though, it's the kind of thing that keeps people fascinated for years.

So it's no surprise that earlier this week, a Redditor going by the handle airlaflair posed the following question: "What is the greatest unsolved mystery of all time?" Over the course of a single day, the AskReddit thread received nearly 5,000 comments. The usual suspects make appearances, of course; one user talked about the Black Dahlia murders, for example, and another brought up Amelia Earhart's disappearance. Anyone with a passing interest in true crime (actually, anyone who paid attention in history class) probably has a general idea of what makes those mysteries, well, mysterious.

But the thread also pointed out several lesser-known mysteries as well — not just famous disappearances, but art heists and the fate of ancient libraries. Head over to the Ask Reddit post to read the comments for yourself, but if you're short on time, here are six highlights.

1. Isabella Stewart Gardner Museum Theft

What's the Isabella Stewart Gardner Museum theft of 1990? Oh, just the biggest art heist in American history. According to the Boston museum's website, the theft took place early in the morning on March 18, 1990, when two men disguised as police officers told security guards they were responding to a call. Once they were inside, they tied up the two guards and stole 13 works of art, including three works by Rembrandt van Rijn, five by Edgar Degas, and a Johannes Vermeer masterpiece, The Concert. According to Penn State, the thieves' haul was worth around $500 million.

In 2013, more than 20 years later, investigators announced they believed they had identified the thieves as "members of a criminal organization with a base in the Mid-Atlantic states and New England." The paintings themselves, however, have never been located. The museum continues to offer a $5 million reward for their return.

2. The Decline Of The Indus Valley Civilization

According to PBS, archaeologists in the 1920s came across the remains of two cities in modern Pakistan and northwest India: Harappa and Mohenjo-daro. These cities, the largest of the Indus Valley Civilization, displayed advancements like urban planning, drainage and sewage systems, and a merchant social class, not to mention the writing system that remains untranslated today. Around 1800 B.C.E., however, things began to decline. According to the BBC, citizens stopped repairing homes, drains became blocked, and trade with Mesopotamia halted.

Historians aren't quite sure what happened, but there are a few theories. One theory believed that the collapse was due to an due to an invasion, but it fell out of favor. Today, many researchers attribute the civilization's decline to climate change, disease, and other similar factors. Until their writing system is deciphered, however, it's slow going.

3. The Zodiac Killer

You had to know the Zodiac Killer would make an appearance. Despite the meme accusing Ted Cruz of allegedly being the notorious serial killer (he's not), the Zodiac was never actually identified. If you need a quick refresher on the subject, here's the short version: In 1968, a teenage couple was shot and killed in 1968 in Vallejo, California. This turned out to be the first incident in a series of murders taking place between the late '60s and early '70s. At least five were killed and two more injured, but the killer claimed to have murdered 37 overall. Oh, yeah — he wrote handwritten letters to police bragging about his actions. In 1974, however, the letters stopped, and the murders appeared to as well. To this day, the Zodiac Killer has never been identified.

4. The Beaumont Children

On January 26, 1966, three children went missing on a trip to the beach in Adelaide, Australia, in what remains one of the most infamous cold cases in the country's history. The children took a bus to a nearby resort that morning; at 9 years old, the eldest child was instructed to look after her younger siblings. They were expected home by 2 p.m., but the children never returned. After the parents reported the disappearance, witnesses claimed that the three siblings had last been seen with a man in his late 30s. The following investigation was heavily publicized and took unexpected twists, including the involvement of a psychic and a possible connection to murders of other children.

Despite national scrutiny of the case and hundreds of tips from callers, however, the case was never solved.

5. D.B. Cooper

The Reddit comment above doesn't quite get the facts correct, but the D.B. Cooper case is fascinating. Here's what really happened: On November 24, 1971, a man who called himself Dan Cooper hijacked a passenger plane with the threat of a bomb in his briefcase. He forced the plane to land in Seattle, where he allowed all the passengers to exit.

After demanding $200,000 in a knapsack and several parachutes, he told the pilots to keep the doors unlocked and fly a specific route to Mexico City at a low altitude. At some point on the way to Reno, Nevada, Cooper parachuted out of the plane — a jump that was missed by officials in five different planes following the hijacked aircraft. The FBI points out that Cooper was unlikely to survive the jump, but no body was ever found and no suspect prosecuted.

6. The Library Of Alexandria

People aren't the only ones who disappear. The library of Alexandria, a Hellenistic city located in northern Egypt, is believed to have been the most extensive library of its time, with hundreds of thousands of scholarly works. However, no physical evidence remains of its existence. The cause of its destruction has been attributed to causes ranging from an enormous fire to mere government budget cuts, but either way, we'll never know exactly what the library contained.

Images: Wikimedia Commons (2)