1. Part Marshland, Part Urban City

Thonis-Heracleion acted as Ancient Egypt's port city to the Mediterranean. Sometime before 800 C.E., the city disappeared, and it became lost to time. Some believed it may have never even truly existed.

Those who thought the city may have been a myth were proven wrong when it floated back onto the map in the early 2000s, when its ruins were discovered under the sea off the coast of Egypt. The entire city was buried underwater, and divers uncovered bridges, 16-foot statues, animal sarcophagi, ruins of temples, shards of pottery, coins, jewelry, oil lamps, and more.

Archaeologists believe that the once-bustling city ended up beneath the Mediterranean Sea as a result of the slow caving-in of the soil, possibly triggered by an earthquake or rising sea levels. Anything is possible, but our money is on soil liquification—AKA quicksand.

2. Huge Stone Jars

Immense stone jars dating back to the Iron Age litter the plains of a remote mountainous area of Laos, and no one knows why they're there. These aren't just ordinary little cookie jars. The largest ones range in size up to 10 feet tall, weighing around 32 tons. They are separated into 90 different, seemingly random groups, ranging in number from one to more than 400 jars.

Bones and other human remains surround the area of the jars, leading some researchers to believe it had been a burial site for an ancient civilization that would first leave corpses to partially decompose in the jars, then bury their bones into the ground.

However, locals have their own theories. Some believe that the jars were used to brew rice wine, to be drunk by giants, when celebrating victories against their enemies.

Archaeological research in the area has been difficult to maintain over the years, due to the use of cluster bombs by the U.S. military during the Vietnam War, which destroyed some jars and left behind undetonated bombs, making the area extremely dangerous.

3. Roman Jars in Brazil

Something is hiding in the waters off Rio de Janeiro. At the bottom of Guanabara Bay lies hundreds of barnacle-covered amphoras—ancient jars commonly carried by Roman ships in the second century B.C.E., and discovered by Robert Marx in 1982.

So what are a bunch of ancient Roman jars doing in Brazil, when the first known Europeans to make their way there didn't show up until the late 1500s? The Brazilian Navy closed the bay to underwater explorers and issued a government order to prohibit Marx from entering the country again. Some think Brazil didn't want Roman artifacts to be found, as it would mean that the Portuguese weren't the first Europeans to reach Brazil.

4. Nazca Lines

Move over, crop circles, it's the Nazca Lines. This collection of geoglyphs is located on the Peruvian coastal plain. The designs etched into the ground spanned more than 185,000 acres. They are made up of over a thousand geometric designs, and straight lines—some stretching over 12 miles long. About 70 shapes of flora and fauna that measure up to 1,200 feet, and even a humanoid figure nicknamed "The Astronaut."

The lines are believed to have been created by the ancient Nazca culture starting around 500 B.C.E., though researchers don't know exactly why they created them.

5. Cleopatra's Tomb

Everyone knows where Cleopatra was between 69 to 30 B.C.E. But does anyone know where she is now? The location of Cleopatra's tomb remains a mystery to this day.

Some believe she was buried in Alexandria, where she was born and ruled. But the city was nearly leveled by a tsunami in 365 C.E. Others believe she was buried on the Nile Delta, in the temple of Taposiris Magna, where two mummies covered in gold leaf of high status individuals from Cleopatra's time were uncovered. Researchers are only certain of one thing: they can't definitively say it's the tomb of Cleopatra.