From a missing plane to a vast city hidden beneath Earth, 2014 left many stones unturned. Although we made many discoveries over the course of 12 months, some of them led to even more puzzling questions.

See also: The biggest stories of 2014

Will 2015 be the year our search for answers comes to an end? Here are the mysteries the world will try to unravel in the new year.

MH370

What happened to the plane that vanished without a trace?

A crewman on a Royal New Zealand Air Force P-3 Orion Rescue Flight 795 searches for debris from the missing Malaysia Airlines Flight MH370 on April 1.

A Malaysia Airlines flight vanished without a trace while en route from Kuala Lumpur to Beijing on March 8. The weather was clear and there was no distress signal from the cockpit when the plane disappeared from radar screens. A massive search operation combed the Indian Ocean, but the Boeing 777 was never found.

After more than nine months, not a single piece of debris from MH370 has turned up. Despite numerous conspiracy theories, we still don't know what happened to the plane. All 239 people on board are presumed dead.

ICELAND'S VOLCANO

Why is Bardarbunga still erupting?

Steam rises into the air near the Bardarbunga Volcano on Sept. 2, 2014. Image: ARCTIC-IMAGES/CORBIS

The eruption of the Bardarbunga volcano in Iceland began at the end of August and continues to this day. The volcano has been unique in three main ways: the enormity of its lava field, the amount of hazardous sulfur dioxide gases it has released and the astonishing amount of sinking that has taken place in its crater.

Bardarbunga created a 32 square mile lava field, which is the largest in Iceland since the Laki eruption in 1783, and the Icelandic Met Office says it is “probably the third largest lava field on Earth” since 1783. The gases released by the volcano have affected all of Iceland at one point or another, the first time that has happened in 150 years. The Met Office says the amount of sulfur released by this volcano may have exceeded that of any volcano on Earth of this particular type.

The eruption has caused the Bardarbunga caldera to sink to 184 feet, swallowing a GPS instrument that had been installed to measure the sinking rate. This is the largest amount of sinking, or subsidence, at any caldera in Iceland in modern times.

ADNAN'S STORY

Is the man spending life in prison the real killer of Hae Min Lee?

Adnan Syed, who is featured in the popular Serial podcast, was convicted of killing Hae Min Lee in 1999.

A 1999 murder case was resurrected and put under the microscope this year thanks to Serial, a nonfiction podcast that unravels a story week by week.

Currently, Adnan Syed is serving a life sentence for murdering his ex-girlfriend Hae Min Lee when they were in high school. When Serial host Sarah Koenig started investigating the case a year ago, the strange story surrounding the murder of the Baltimore high school student, a lot of things didn't add up. There were no eye-witnesses tying Syed, who has been trying to prove his innocence for years, to the crime, and Syed's attorney, Cristina Gutierrez, failed to interview a witness who said she was with him at the time Lee was killed.

Ultimately, a Maryland appeals court will decide the man's fate. A hearing scheduled for January represents what Syed's lawyer, C. Justin Brown, said is the man's "last best chance" at freedom.

SIBERIAN SINKHOLES

What caused three deep holes in Russia's north?

A view of a sinkhole, which stretches 20 meters by 30 meters, at the Solikamsk-2 mine in Russia’s Perm region.

A 115-foot hole was discovered in Siberia's Yamal peninsula — a name that means "end of the world" in the language of the area's indigenous inhabitants — after an unexplained explosion in July. Two other mysterious sinkholes popped up in Russia's north shortly after. Scientists believe gas explosions beneath the surface caused the sinkholes, but they haven't been able to confirm that theory.

In November, a team of scientists, a medic and a professional climber plunged into the sinkhole to learn more about it, but the cause of the deep holes is still unknown.

SADDLE RIDGE GOLD

Was the $10 million treasure actually from a 1901 heist?

Inside a Saddle Ridge Gold Coin Can. Image: Kagin's

Buried for more than a century, 1,400 gold coins — worth an estimated $10 million — were unearthed in a California backyard in 2014. The middle-aged California couple who discovered the eight decaying tin cans full of gold remain anonymous.

Where did the gold come from?

The tale grew even more curious as we discovered that $30,000 worth of similar gold coins — that amount would be in the millions in today's dollars — had been stolen from the U.S. Mint in San Francisco in 1901. Soon we were all playing a game of whodunit.

Former San Francisco Mint clerk Walter N. Dimmick was charged with that theft, but, by the prosecution’s own admission, he was convicted on wholly circumstantial evidence. Despite the fact that the Secret Service searched all over California for those coins, they were never found.

But maybe that couple living in Northern California finally did.

CELEBRITY PHOTO HACK

Who is behind the attack that exposed A-listers' most private messages?

Jennifer Lawrence, pictured here on Nov. 17, was one of several victims of the iCloud hack. Image: Jordan Strauss/Invision/AP/Associated Press

Beginning in late August an anonymous 4chan user began posting hundreds of never-before-seen private photos of celebrities that were stolen from their hacked iCloud accounts. The victims were mostly female and high-profile: Jennifer Lawrence, Scarlet Johansson, Kim Kardashian, Kate Upton and McKayla Maroney are just a few women whose accounts were compromised.

Apple confirmed the hackers had obtained the images by means of a "targeted" attack, though we still don't know who — or how many hackers — was involved. The FBI said it was looking into the case.

ALIEN MOON

Did scientists really discover an exomoon for the first time?

Researchers have detected the first "exomoon" candidate — a moon orbiting a planet that lies outside our solar system.

Scientists think they found the moon of an alien planet, which would be the first time we've ever seen an "exomoon." The problem is that we'll never really know if that's what astronomers saw.

Researchers discovered the possible exomoon using gravitational microlensing — a technique that detects distant objects regardless of the light they emit — to take advantage of chance alignments between stars. When a foreground star passes between Earth and a more distant star, the closer star can act like a magnifying glass on the more distant one. If that star has a planet whizzing about it, the planet will brighten or dim the light of the distant star. But in some cases, the distant object could be a free-floating planet instead of a star, giving scientists the ability to measure the mass of the planet relative to its companion.

In this case, scientists could have spied a small star circled by a planet 18 times Earth's mass — or they could have spotted, for the first time, a planet bigger than Jupiter teamed up with a moon weighing less than Earth.

But because this encounter was completely random, it's impossible to know if we finally laid eyes on an exomoon. So, the search continues.

BENEATH STONEHENGE

What is the sprawling structure hidden under the world's most famous standing stones?

Researchers uncovered 17 previously unknown ancient monuments under Stonehenge. Image: Stonehenge Hidden Landscapes Project

Unprecedented underground mapping technology revealed that Stonehenge did not always stand alone. Beneath the 5,000-year-old stone structure is an archeological treasure trove made up of thousands of monuments that include shrines, burial mounds, deep pits and other unexplained features. Stonehenge was previously thought to be a standalone, isolated structure, so this discovery has thrown researchers for a loop.

"Stonehenge may never be the same again," said Vince Gaffney, the project's lead researcher.

And this is just the beginning — the vast majority of information from the site has yet to be analyzed.

"What you’re hearing about is simply a mere little finger of the amount of data that we have now," Gaffney told Mashable. "We think we’ve got at least another year’s work on this."

SONY HACK

Was North Korea really behind the massive cyberattack on Sony?

An undated handout picture released by the North Korean Central News Agency (KCNA) on April 27, 2014 shows North Korean leader Kim Jong-un looking at a computer screen. Image: EPA/KCNA

Sony is still reeling from a massive hack that exposed embarrassing company secrets and led the the cancellation of a movie release. The FBI blamed the hack on the North Korean government, saying the attack was in retaliation for The Interview, a Sony film starring James Franco and Seth Rogen that chronicles the assassination of Kim Jong-Un. However, not all security experts are convinced that the link to North Korea — specifically, the North Korean government — is accurate, especially in the absence of more concrete evidence.

"One of the biggest mistakes is that because an attack can be traced to the North Korean Internet that somehow means it's the North Korean government. That's a false assumption because the North Korean Internet is basically provided by outside companies, in this case a Thai company," Jeffrey Carr, cybersecurity expert told Mashable. "Nothing presented excludes alternate scenarios, so why jump to the most serious one?"

MEXICAN STUDENTS

What happened to the 43 students who went missing?

Chairs sit in a pile int he corner of a class room that has been cleared out in the Ayotzinapa normalista school.

Nearly four dozen high school students disappeared in Mexico after police stopped them from traveling to a protest. The cops then reportedly handed over the students to a drug gang, who investigators say killed them. However, their bodies have never been found.

Nearly 80 people have been arrested following the kidnapping, including a mayor, his wife and a local police chief. But without their bodies, the case remains unsolved.

TWIN TWISTERS

What caused the double tornadoes that struck Pilger, Nebraska?

Two tornadoes approach Pilger, Nebraska, Monday June 16, 2014. Image: Eric Anderson/Associated Press

The beastly twin tornadoes that leveled the small town of Pilger, Nebraska in June, killing two and injuring nearly two dozen, were a horrifying, relatively rare phenomenon. Both of the twisters were rated at EF-4 intensity on the Enhanced Fujita Tornado Damage Scale, with winds as high as 200 miles per hour. One of them went directly through Pilger, while its twin spun itself out to the east of the town.

The sibling tornado damage tracks were visible from space, where a NASA satellite spotted what look like nearly identical scars on the Earth's surface, separated by about two miles.

There are many things scientists still don’t know about tornadoes. For example, they still don’t know exactly why some severe, rotating thunderstorms, known as supercells, produce tornadoes while other nearly identical storms do not. The Pilger tornadoes demonstrated another unknown — how some storms can spawn multiple violent tornadoes at the same time.

Greg Carbin, a meteorologist at the Storm Prediction Center told Mashable after the event that the tornadoes were unusual since we rarely see such a “vivid example of a complex tornado.”

NIGERIAN GIRLS

Where are the schoolgirls who Boko Haram kidnapped?

Martha Mark, the mother of kidnapped school girl Monica Mark cries as she displays her photo, in the family house, in Chibok, Nigeria. Image: Sunday Alamba/Associated Press

Nearly 300 schoolgirls were kidnapped in April from the Government Secondary School in Chibok, Nigeria, by a radical Islamic group called Boko Haram. In early May, the hashtag #BringBackOurGirls began to spread on Twitter, raising the world's awareness of the mass abduction.

In May, Nigeria's chief of defense staff, Air Marshal Alex Badeh, said the country's military had located the schoolgirls, but using force to free them is currently out of the question. Badeh did not specify the girls' location, and a U.S. Defense Department spokesperson said it could not confirm Nigeria's report.

By year's end, a majority of the girls remain missing.

STEALTHY SUBMARINE

Did the submarine spotted near Sweden come from Russia?

Swedish navy corvette HMS Visby searches for a submarine in the Stockholm Archipelago, Sweden, on Oct. 19.

Sweden's top general recently said the nation's territory was "unacceptably violated" in October, but he couldn't say by whom.

The Swedish military found traces of a mysterious submarine lurking in the water near the nation's capital, Stockholm, and many officials pointed the finger at Russia. But after a days-long search operation around the Stockholm Archipelago's 30,000 islands, Swedish officials called off the hunt. They said the submarine was foreign, but its country of origin remains unclear.

Additional reporting by Andrew Freedman, Colin Daileda and Lance Ulanoff