Biggest Mysteries of 2014

— -- Horrifying crimes and the bizarre behavior of a strange world leader captured the public's imagination and piqued curiosity.

One story stood apart from all the rest, however, as neither science nor technology could help solve the world's biggest mystery: How could a jetliner full of people, in our intensely scrutinized and electronically tracked world, simply disappear?

Some questions, like the whereabouts of the missing plane or the schoolgirls kidnapped by the militant group Boko Haram, remain unanswered, while there are other cases where the end result is known but major pieces of the story remain a mystery.

Have a look back at some of the most mind-boggling stories of the past 12 months.

1. Where is the missing plane?

Conspiracy theories ran rampant for much of the first few months after Malaysia Airlines Flight 370 vanished from the sky on March 8. While some explanations (like the situation being a real-life version of the television show "Lost" or the 747 disappearing into a fatal black hole) have been ruled out by common sense, science has not been able to find any trace of the Beijing-bound plane or the 239 people on board. Multiple international searches have yielded no concrete answers, but the most conclusive flight tracking data strongly suggests that the plane crashed somewhere in the vast Indian Ocean.

2. Where are the missing girls? Boko Haram and #bringbackourgirls

During a night in mid-April, 276 teenage girls were kidnapped by an Islamic jihadist group in Chibok, Nigeria, and the news of their capture prompted international outrage. Word of the attack spread via social media with the hashtag #bringbackourgirls. About 50 of the girls have been able to escape their captors, who have been identified as members of the militant group Boko Haram, and though there have been rumors that the girls could be released as a result of an exchange with the Nigerian government, no such deal has come to fruition. An estimated 219 girls are still missing.

3. Where was Kim Jong-un?

So much about North Korea is a mystery on the part of the autocratic government, but the rogue nation came under intense international scrutiny after ruler Kim Jong-un had not been seen in public for 40 days this fall. The absence was so pronounced because he is typically a constant presence in the state-controlled media. Outside experts noticed a slight limp in footage from one of his last sightings and combined with his recent visible weight gain, the widely-accepted conclusion pegged the dictator as having had some kind of medical operation that forced him out of sight during a recovery period. Kim Jong-un -- who is believed to be 31 years old -- returned on Oct. 13 and images released by the state media showed him smiling while walking with a cane, though there was never any formal explanation for his absence.

4. What happened to Hannah Graham?

The disappearance of 18-year-old University of Virginia student Hannah Graham came to a tragic conclusion when state police found her skeletal remains more than a month after she was last seen. Graham texted friends in the early hours of Sept. 13 and police used surveillance footage and witness accounts to place her with Jesse Matthew, a hospital technician. After announcing him as a person of interest, he fled the state and was later caught in Texas. He has now been formally connected to at least two other abductions in Virginia in the past nine years though he remains in jail and has not yet entered pleas on any of his charges.

5. How did Eric Frein avoid capture for so long?

After allegedly killing a Pennsylvania state police trooper, 31-year-old Eric Frein was able to hide from local and federal authorities for nearly seven weeks. The survivalist hid in the sprawling woods of the Pocono Mountains and experts believe that he was able to evade capture for so long because he had planned ahead. Searches of his computer showed that he had prepped by researching SWAT raid tactics, police cell phone tracking technology and how best to hide caches -- small duffle-type bags that hunters use to hide food and supplies in the wilderness. He was finally found at an abandoned airport hangar the day before Halloween and surrendered without a struggle. All told, the manhunt lasted 48 days and cost the state more than $11 million.