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Science is all about the pursuit of truth. New discoveries can provide answers, but they can also open the door to new questions.

Explore some of the mysteries left unsolved at the end of the year.

Top Myths Busted in 2013

David A. Aguilar (CfA)

Kepler-78b shouldn't exist. An Earth-sized planet with a rocky surface and an iron core, Kepler-78b is so close to its parent star that it completes an orbital revolution every 8.5 hours.

Kepler-78b is a curiosity because scientists have no way of explaining how the lava world could have formed given its proximity to its parent star.

Kepler-78b: Mystery Exoplanet Shouldn't Even Exist

Bailey et al./UA/MagAO

Exoplanet hunters had another mystery on their hands in 2013: a gas giant, HD 106906b, 11 times the size of Jupiter with a peculiar orbit. Unlike Kepler-78b, which is very close to its parent star, HD 106906b is 650 AU (astronomical unit) -- the measure of the distance between the Earth and the sun -- away from its parent star.

In fact, that's such a great distance that the orbit is larger than what astronomers once thought possible. Like Kepler-78b, the unusual distance means scientists do not yet how this exoplanet formed.

'Impossible' Alien World is Impossible. Yet There It Is

Corbis

Human ancestors were a promiscuous bunch. Ancient Homo sapiens mated not only amongst themselves, but also interbred with Neanderthals, a line of humans known as Denisovans, and a mystery lineage of humans. The unknown, extinct population isn't yet in the DNA record, as reported by LiveScience's Stephanie Pappas.

Given the different hominid species around at the time, Mark Thomas, an evolutionary geneticist at University College London, described it as a "Lord of the Rings-type world."

Are You Related to Neanderthals?

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Do you hear that? That steady, droning, persistent sound that creeps at night.

If you are hearing things, you're not alone. Since the 1950s, reports have been coming in from around the world of people hearing what is known as "the Hum." As LiveScience's Marc Lallanilla reports, only about 2 percent of the population lives in a hum-prone area. It's louder at night than it is during the day, and typically heard in rural and suburban areas.

Mysterious Sounds Reported Around the World

Durham University

Christopher Columbus wasn't the first European to set foot in the New World. The Vikings preceded him. And in 2013, we learned of the arrival of a mysterious group of European settles to the "steps to the Americas" 300 to 500 years before the Vikings arrived in the New World.

Scientists had previously thought the Vikings were the first arrivals to the Faroes in the ninth century. Discovered at an archaeological site of Á Sondum on the island of Sandoy, researchers found evidence of human settlement in patches of burnt peat ash.

Although investigators have yet to discover clear evidence of the group's identity, possibilities include religious hermits from Ireland, late-Iron Age colonists from Scotland or pre-Viking explorers from Scandinavia.

Mystery Settlers Reached 'Step to Americas' Before Vikings

Corbis

Five hundred years after she had her portrait painted, we're still waiting to find out he identity of the woman in Leonardo da vinci's famous masterpiece, "Mona Lisa."

Lisa Gheradini Del Giocondo, the wife of a rich silk merchant, has long suspected of being the model behind Mona Lisa, and DNA tests of several skeletons found under a Florence convent will confirm whether one of them is Gheradini.

If one of the skeletons is Gheradini, who died in 1542, researchers plan on commissioning a facial reconstruction to determine any similarities between the skeleton and the portrait.

Mona Lisa Mystery: Answers Soon?

Shmuel Marco

A giant circular stone structure was discovered in the Sea of Galilee following a sonar survey, and archaeologists have no idea what it is or who put it there.

Weighing an estimated 60,000 tons, the cone-shaped structure is made of an "unhewn basalt cobbles and boulders." Aside from its proportions -- 32 feet (10 meters) high with a diameter of 230 feet (70 meters) -- little else is known about the structure. Researcher Yitzhak Paz, of the Israel Antiquities Authority and Ben-Gurion University, has suggested it could be more than 4,000 years old, citing other megalithic phenomena near the site.

Stone Mystery in Sea of Galilee

Why would anyone leave a treasure trove in a garbage dump?

Located near the ancient city of Apollonia-Arsuf at a site north of Tel Aviv, a hoard of 400 Byzantine coins, 200 intact Samaritan lamps and gold jewelry was found in an ancient garbage pit amid animal bones, pottery and glass fragments. The majority of the objects date between fifth and seventh centuries.

Mystery of Byzantine Garbage Pit

Troy Alexander / Tambopata Research Center

Several unusual-looking web structures were discovered in the Peruvian Amazon, and until just recently scientists didn't know what it was. About two centimeters (0.8 inches) wide in diameter, the formation is a tiny spire surrounded by a webby fence.

The entomologist who found it thought it might be a new type of fungus or a spider egg carrier. He and a team of researchers kept watch on the structure, and just as they were about the leave the Amazon without figuring it out, they found three spiderlings that emerged from it and went scrambling off.