Unsolved Mysteries of Ancient Ruins Around the World

The pyramids of Egypt, the statues of Easter Island, and the monoliths of Stonehenge have stood for millennia. These icons of the ancient world and others like them have attracted, bewildered, inspired, and intrigued people for as long as they’ve stood. Even today, historians and archaeologists don’t always know why or how these monuments were built or what purposes they served. (Which only adds to their mysterious appeal.) Take a trip around the world and visit some of the most mysterious ancient sites on earth.

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Centuries before Europeans set foot in North America, a city of as many as 20,000 people spread out on the Mississippi River floodplains near present-day St. Louis. Founded sometime around 1000 A.D., the inhabitants built earthen mounds — about 120 total across a site of 8 or 9 square miles, historians estimate — including the tallest, Monk’s Mound, which rises 100 feet and covers 14 acres. By the mid-1300s, however, the metropolis is thought to have been depopulated, perhaps due to environmental changes. Several of the mounds have been destroyed over time by agriculture and development, except for the 2,200-acre state historic site that contains about 70 documented mounds, including Monk’s Mound.

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This snake-shaped earthen mound, located about 70 miles east of Cincinnati, stretches more than 1,350 feet from the open-mouthed head to its coiled tail. Archaeologists estimate that it may have been built as long ago as 800 B.C. or as recently as 1100 A.D., but precisely when — and by which Native American culture — remains unclear. Today, the site is a state park and a designated National Historic Landmark.

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Built by prehistoric communities that inhabited the Sonoran Desert from about 300 to 1500 A.D., this site near Coolidge, Arizona, contains ruins of a walled compound, including the large adobe structure for which it is named. Archaeologists think the building, which ranges from three to four stories in height, may have been part of a larger agrarian settlement. The site was named a national monument in 1918. 

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Only viewable from the air and higher elevations, these epic figures (called geoglyphs), geometric shapes, and lines  — at least 1,200 by one estimate — cover a roughly 1,900-square-mile portion of Peru’s Nazca Desert. They were constructed between 500 B.C. and 500 A.D. by the Nazca people, a prehistoric culture, and thought to have ritualistic astronomical significance. Archaeologists are still discovering new geoglyphs, thanks to drone technology; in late 2020, the Peruvian government announced the discovery of a cat-shaped glyph measuring some 121 feet in width.

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Perhaps the most iconic of the ancient world’s monuments, there’s a lot that is known about the three Great Pyramids — and plenty more unknown. The pyramids, part of a UNESCO World Heritage site on the Nile River near Cairo, were built during the Fourth Dynasty (roughly 2575 to 2465 B.C.), each a monumental tomb for one of the dynastic kings. The largest of these, the Pyramid of Khufu, rises about 450 feet. Among the mysteries still unsolved: precisely how the limestone and granite structures were constructed, how many people it took to build them, and what treasures these structures — looted centuries ago — once contained.

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Another icon of the ancient world, the stone slabs of Stonehenge were erected about 2500 B.C., although the site itself had been in use for at least 500 years before. Historians still debate how the large sarsen stones, some of them weighing upwards of 300 tons, were transported from their likely origin 20 miles away. Another mystery: Why it was built in the first place. Historians have speculated that it may have once been a site for religious ceremonies or designed to be a solar calendar that tracked the passage of the seasons.

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This series of artificial islets and canals, the oldest of which dates to the 13th century, lies just off the coast of Pohnpei Island. Built atop a coral reef from columns of black basalt — some of them weighing as much as 100,000 pounds — the complex was once home to an estimated 1,000 people. How Nan Madol’s builders were able to cut, transport, and fit these stones is unknown. The city served as the political and religious center of power for the Saudeleur dynasty until the 1700s, when they were overthrown and the area gradually abandoned. 

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Ranging in size from a few inches to more than 6 feet in diameter, about 300 of these stone spheres have been unearthed in several locations in southwestern Costa Rica and nearby Isla del Caño beginning in the 1930s. Archaeologists attribute them to the indigenous cultures that lived in this region of present-day Costa Rica between 500 and 1500 A.D., and some people speculate they may have been indicators of wealth or power.

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Another mysterious scattering of sculpted stone lies in northern Laos. Thousands of Iron Age carved jars ranging in size from 3 to 10 feet tall, some of them with matching lids, have been identified at nearly 100 sites. They date to between 500 B.C. and 500 A.D., although archaeologists have not been able to pinpoint when they were created or for what purpose. It is speculated that the jars were perhaps used as part of the burial process for local cultures. Much of the region was carpet-bombed by U.S. forces during the Vietnam War, and the presence of unexploded ordnance limits new archaeological research.

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Some 900 of these stone humanoid monoliths dot Easter Island in the South Pacific. Carved from local volcanic rock by the indigenous Rapa Nui people between 1250 and 1500 A.D., they range in size from about 3 feet to 71 feet in height. Archaeologists have speculated that the monoliths represent powerful rulers or ancestors of the Rapa Nui people, but research from 2019 suggests they may also have served as markers for fresh water on the island, where such resources were scarce. Another puzzle: how the multi-ton stones were transported across the island from quarry to installation site and how many people it took to install these monoliths.

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