We're always interested in compelling true crime stories, but the best ones leave us with more questions than answers.

These 10 unsolved crimes are a great puzzle for your imagination, if you don't mind some grizzly stories. We may never know the truth about them, but there are still cash rewards waiting to be claimed for most of these cases.

10. D.B. Cooper

In 1971, a man using the alias Don or "D.B." Cooper bought a one-way airline ticket to Seattle. Once the plane was in the air, he claimed he had explosives in his suitcase and hijacked the plane.

After ordering the pilots to land, Cooper was given $200,000 by police, then ordered the plane to take off again. He finally jumped out of the plane - with a parachute and the money - somewhere between Seattle and Reno.

While a handful of clues have turned up, after interviewing more than 1,000 suspects the FBI's file on the Cooper skyjacking is still open.

9. The Long Island Serial Killer

By chance, an officer training his canine partner in 2010 discovered a woman's body dumped beside Long Island's Ocean Parkway. The body was stuffed in a burlap sack, and within two days police found three more just like it.

Authorities soon realized they had a serial killer on their hands, but they've never identified the man suspect of murdering as many as 16 women over almost two decades. The killer is also known as the "Craigslist Ripper" for his habit of targeting escorts on the popular website.

The $25,000 reward for information that puts this killer behind bars is still unclaimed.

8. The Gardner Museum Robbery

In the early hours of March 18, 1990, two police officers responded to a distress call at Boston's Isabella Stewart Gardner Museum. After museum guards let the men inside, they revealed themselves to be thieves in disguise.

After handcuffing the guards, the thieves walked out with 13 pieces of art worth a combined $500,000, making the heist the most valuable theft of private property ever.

Last year, the museum offered a $10 million reward for information about the heist, but there's still no sign of the two robbers or any of the art.

7. The Black Dahlia

Actress Elizabeth Short was found murdered in 1947, with her body dumped in a vacant lot outside Los Angeles. Police were stunned to find Short's body had been cut in half, and had its blood drained out before it was dumped.

While more than 150 suspects were interviewed - and some even turned themselves in - no one was ever arrested for Short's murder.

6. The 300 Million Yen Robbery

On December 10, 1968, four employees of a Japanese bank were traveling through Tokyo in a company car loaded with 300,000 million Japanese yen. The huge load of bank money and employee bonuses would be worth $5.7 million today.

A police officer on a motorcycle warned the men their car had been strapped with a bomb, which fit with threatening letters the bank had received days earlier. The officer tried to disarm the bomb under the car, but smoke and flames started pouring out from underneath it.

As the bank employees ran for cover, the fake police officer drove away with their money. The bomb scare was part of the robber's plan, and no trace of the money was ever found.

5. The Palo Verde Derailment

On October 9, 1995, an Amtrak train outside Palo Verde, Arizona crashed. One train worker was killed and 78 people were injured as the train flipped off a bridge into a riverbed. Investigators found that the track had been sabotaged by someone who knew how not to set off Amtrak's security systems.

Notes left to the FBI and the ATF at the crime scene claimed a militant group called "Sons of the Gestapo had derailed the train to protest the government.

Some think the "Sons of Gestapo" notes were meant to be cover for a freight train robbery, or an alibi for a disgruntled Amtrak employee. In any case, the saboteurs behind the derailment have never been arrested.

4. The Women of Ciudad Juarez

This is not just one unsolved and mysterious crime. Over the past few decades, hundreds of women have been killed or disappeared in the Mexican border city of Juarez. While many deaths are blamed on gang wars and sexual assaults, some say that doesn't explain every case.

While there are rumors of a serial killer preying on the women of Juarez, most experts point fingers at Mexico's shoddy justice system, and the many temporary factor workers who flock to Juarez each year, who make easy targets for killers.

3. The Murders of Tupac and B.I.G.

Less than six months apart, two of the biggest names in hip hop were murdered in drive-by shootings. For more than two decades, their fans have been searching for answers.

The Notorious B.I.G. was shot four times in a drive-by on the streets of Los Angeles, and his murder has been pinned on everyone from rival gang members, to music producer Suge Knight, and even the L.A.P.D.

Likewise, Tupac's murder by drive-by shooting in Las Vegas is still a mystery. Were these musicians killed because of cross-country gang rivalries, targeted assassinations, or just bad luck? We may never know.

2. The Zodiac Killings

"I like killing people because it is so much fun it is more fun than killing wild game in the forrest because man is the most dangeroue anamal."

That was the chilling message sent to a trio of San Francisco newspapers by the mysterious Zodiac Killer. The serial murdered terrorized Northern California in the '60s and early '70s.

While there are just seven confirmed Zodiac victims, the killer claimed to have secretly murdered as many as 37 people. Famously, he also taunted police in a series of coded messages - some of which are still unsolved.

Researchers are still suggesting new theories about this case, but after 50 years it's likely we'll never know the whole truth.

1. Jack the Ripper

Long before the phrase "serial killer" even existed, London's citizens were terrorized by the killer who stalked the street of Whitechapel looking for his next victim. While he was known as "Leather Apron" at the time, the more chilling "Jack the Ripper" has stuck throughout the years.

The Ripper was known for targeting Whitechapel's prostitutes and cutting out their organs (hence the nickname). While there are five famous Ripper victims, some experts pin 11 murders (or even more) on the famous killer

There's an entire study called "Ripperology" devoted to studying the killer and trying to solve the mystery of his crimes. With more than 100 competing theories about the Ripper's true identity, maybe you can be the person to finally crack the case.

Have any theories about these cases?