8 Amazing Structures With Mysterious Origins

How prehistoric people carved stone into intricate shapes and sculptures or moved boulders that weigh up to 30 tons and stacked them on top of each other, even before the invention of the wheel, is one of the greatest mysteries of human history. Certain ancient landmarks around the world beckon the age-old question, "how did this get here?"—and equally perplexing, "why?" Perhaps their centuries-old secrets make them even more attractive.

From Easter Island's famous moai statues to England's Stonehenge and Mexico's giant Olmec heads, here are eight structures with mysterious origins that continue to stump experts even in the modern age.

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Nan Madol

In the South Pacific nation of Micronesia, Nan Madol is an imposing stone city that sits on top of a coral reef in a lagoon adjacent to the island of Pohnpei. A network of natural "canals" connects the different islets of this ancient complex. Carbon dating put the earliest settlements in the area at about 1200 C.E., though some archeological finds suggest that people were living on Pohnpei more than 2,000 years ago.

Little is known about Nan Madol's monolithic structures. The large stone blocks of which they're composed are too heavy to have been moved without mechanical aid. Many theories and myths exist about their origins, including local legends involving black magic and hypotheses about a "lost race" that came from a now-disappeared continent. There are also more believable (but unproven) theories that suggest Nan Madol was a royal complex meant to keep the island's elites separate from the commoners.

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Skara Brae

Grass-covered stone settlement on the coast of Scotland — Ashley Cooper / Getty Images

Located on Scotland's rugged Orkney Islands, the moundlike buildings of Skara Brae, a Neolithic settlement, are in great condition considering they're thought to be much older than the Great Pyramid of Egypt. Most estimates put them at 5,000 years old. The settlement has been called the "Scottish Pompeii" because it remains almost pristine despite being long abandoned. Sand blown in from Orkney's coastal dunes has effortlessly preserved it.

The eight dwellings of Skara Brae and the passageways now bring scientists great insight into Scottish life in Neolithic times, but the history of the site remains a mystery. Human remains, carvings, and a bull's head were discovered in a building isolated from the rest of the complex, inspiring theories about ancient religious rituals. Also, it's uncertain whether it was the encroaching dunes or some catastrophic event that caused residents to abandon the village more than 4,000 years ago.

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Newport Tower

Round, stone tower sitting in the middle of a grassy park — FaceMePLS from The Hague, The Netherlands / Wikimedia Commons / CC BY 2.0

The Newport Tower is a circular stone building in Newport, Rhode Island. The most realistic theory explaining its early purpose is that it served as the base of a windmill built in the 16th or 17th century by some of the U.S.'s first European settlers. However, some people hypothesize that it is several hundred years older than commonly thought and provides evidence that someone other than Columbus made the first landing in the New World.

Carbon dating of the foundation and excavations of the surrounding areas seem to support the windmill hypothesis. However, there's also speculation that the tower was some sort of observatory because its windows align with various stars and moon positions, plus the sun during the summer solstice. These odd features have led to theories about Vikings, Chinese sailors, and even that the Knights Templar were responsible for the construction of the tower.

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Easter Island Moai

Up-close view of giant stone sculptures on Easter Island — Volanthevist / Getty Images

The huge-headed statues, called moai, on Easter Island (aka Rapa Nui) in Chile were carved and erected by the island's inhabitants between 1000 C.E. and the second half of the 17th century. Little is known about how how they were carved and moved without the aid of modern equipment, considering the heaviest weighs about 82 tons.

Because the statues resemble those found in other parts of Polynesia, they could represent the islanders' clan ancestors. The early European sailors who landed in Rapa Nui found a civilization in disarray, the few surviving natives either sick or starving. These early encounters showed little evidence of a society advanced enough to carve and transport the moai.

After analyzing the locations of the maoi, however, scientists determined that they were strategically placed near underground aquifers and areas with fresh groundwater. That they were drinking contaminated water would explain why so many perished by the time European settlers arrived.

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Olmec Colossal Heads

Colossal stone head protruding from ground in Mexico — Manfred Gottschalk / Getty Images

Carved from colossal basalt boulders, these head-shaped statues are much older than the more-famous Rapa Nui maoi. Found throughout the Olmec heartland, along the Caribbean coastline of both Mexico and Guatemala, many of the heads are remarkably well-preserved and quite lifelike. They bear distinct features still seen in the Central American descendants of the Olmec.

Each head is carved out of a single boulder, with the smallest example weighing six tons and the largest (an uncompleted head) topping 50 tons. The methods of transporting these boulders remain unclear, and heads found in different areas have slightly different characteristics, supporting the theory that they were modeled on actual people.

These carvings are some of the only clues to the story of the Olmec civilization, which went into steep decline and virtually disappeared more than 2,000 years ago.

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Stonehenge

Large ancient standing stone monuments in a grassy field — Nukorn Plainpan / Getty Images

Wiltshire, England's Stonehenge is another world-famous mysterious structure. Archeologists believe the ring of stone pillars with massive capstones perched on top was erected between 4,000 to 5,000 years ago.

There are no concrete facts about its purpose, but many think it's somehow religious, and the discovery of human remains supports the theory that it was used as a burial ground. The most widely accepted theory is that Stonehenge was a multipurpose religious site for burial and worship of ancestors or deities. The south-central part of England, where Stonehenge is located, was densely populated during the Neolithic Age, and numerous burial mounds and artifacts have been found there.

Experts have confirmed that the larger stones, each weighing up to 30 tons, hail from the Marlborough Downs some 20 miles away and the smaller stones came from southwest Wales. It's still unclear how they were transported.

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Georgia Guidestones

Tall granite monuments with writing inscribed at sunset — Jon Thompson / Getty Images

Not all mysterious sites have ancient origins. One of the U.S.'s strangest structures has been around only for a few decades. Located in northeast Georgia's rural Elbert County, the six Georgia Guidestones (five upright stone walls with one Stonehenge-esque capstone on top) were erected by contractors under the direction and funding of an anonymous party.

Ten guidelines are listed in eight languages on the face of the stones. This cryptic—though not necessarily religious—list has been compared to the Ten Commandments, but perhaps for a post-apocalyptic time. A small tablet near the main structure is inscribed with the size and astronomical alignment of the stones, plus the phrase, "Let these be guidestones to an Age of Reason."

Naturally, the stones have inspired a multitude of conspiracy theories. Some have pointed out that the "commandments" are in line with the teachings of various secret societies around the world.

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Pumapunku

Large megalithic stone with intricate carvings — DmitriyBurlakov / Getty Images

Pumapunku (sometimes written as two words: Puma Punku) is a 1,500-year-old temple that's part of the larger Tiwanaku archeology sites in western Bolivia. Positioned near the famed Lake Titicaca, the stones are at the center of one of South America's most intriguing historic mysteries. They're laid out with accuracy, and the geometrical carvings are extremely precise. The straightness of the cutting is like that achieved in modern times with the use of lasers and computerized equipment.

The quality of the artistry has led to various theories. Some attribute the stones to aliens and others to some super-advanced society that vanished after a sort of catastrophic event. More reasonable Pumapunku theories include the idea that the stones were not natural, but made using some sort of concrete and molds. Others suggest that the ancient craftsmen were just extremely skilled and used methods that historians and archeologists haven't yet discovered.