From castles, cathedrals, and palaces to miles-long bridges, golden temples, and sky-scraping glass towers, the world is full of magnificent feats of architectural engineering. While the purpose of most of these structures is known, there are still plenty of human-made monuments that boggle the minds of even the most acclaimed scientists and archaeologists. Here are 12 such monuments that remain a mystery.
A team from the University of Chelyabinsk discovered this ancient settlement in Russia’s Southern Urals, near the border with Kazakhstan, in 1987. Estimates date the citadel to around the 16th or 17th century BCE. The settlement has a radial setup, with two circular walls encompassed by a defensive system and moat. Inside were around 60 adobe houses. Many agree that it functioned as a fortress, residential community, social center, and temple. Astrologers have also likened it to Stonehenge due to its latitude and location, which would suggest its use as a celestial observatory. Nevertheless, nobody is certain who built it or why it was abandoned.
Carnac Stones (France)
The Carnac Stones are a group of more than 3,000 megalithic standing stones in the French village of Carnac, Brittany. These stones date back to the Neolithic period and were probably erected between 3300 and 4500 BCE. They are one of the world’s largest collections of menhirs, or upright stones arranged by humans. There is also no real evidence to confirm their purpose, but that hasn’t stopped researchers from hazarding guesses. Some theorize they were used as calendars and observatories by farmers and priests. According to Christian mythology, the stones are pagan soldiers petrified by Pope Cornelius. Local folklore implies that the stones stand in straight lines because they were once part of a Roman army. The story goes on to say that the Arthurian wizard Merlin turned the Romans to stone.
Easter Island Moai (Chile)
Over 2,000 miles off the coast of Chile, Easter Island (Isla de Pascua) is the one-time home of a Polynesian tribe called the Rapa Nui. Scattered across the island are around 1,000 moai, giant hand-carved stone statues of human-like figures that are half-buried in the earth. The Rapa Nui landed on the island sometime between 700 and 800 CE and are believed to have started making the moai around 1100 CE. Each moai weighs 14 tons and stands 13 feet tall on average, so it’s hard to imagine how they were transported and hauled into place. One theory is that the islanders used a system of ropes and tree trunks. Their purpose has also been the subject of much debate. To the Rapa Nui, the statues may have stored sacred spirits.
Nazca Lines (Peru)
Southern Peru’s Nazca Desert is covered with hundreds of geometric designs. These ancient geoglyphs range from simple shapes to plants and animals such as a hummingbird, monkey, llama, and whale. The Nazca Lines date back to around 200 to 700 CE, when the Nazca people who lived in the region created them. Researchers have struggled to agree upon the purpose of these giant works of art, particularly since they are best seen from the surrounding hills and by plane. Among many theories are astronomical maps, indicators of sacred routes, and water troughs. An alternative take is that they were created to be observed by deities from the sky.
Stone Spheres (Costa Rica)
In Costa Rica’s Diquis Delta is a group of around 300 polished stone spheres, some just a few inches in diameter and others measuring up to seven feet and weighing 16 tons. Employees of the United Fruit Company stumbled across the spheres in the 1930s, while clearing a jungle to build a banana plantation. Scientists have so far been unable to pinpoint an exact date of their origin, instead suggesting that they appeared anytime between 200 BCE and the 16th century CE. They are commonly attributed to the Diquis people, yet their purpose is a mystery. They might have been property markers of ancient chiefs or remnants of the lost city of Atlantis. Some were even detonated in the hope of finding gold inside.
Temple of Bacchus (Lebanon)
One of the most intriguing Roman ruins on the planet, the Baalbek temple complex in northeast Lebanon features the well-preserved and monumental Temple of Bacchus. The age of the temple is unknown, although it was most likely erected in the second century CE. Historians agree that emperor Antoninus Pius commissioned it in honor of Bacchus, the god of wine and intoxication. What has been baffling archaeologists ever since the temple’s rediscovery in the late 19th century is how the Romans succeeded in building it. It is almost impossible to think that humans could hoist the 42 Corinthian columns of the colonnade (19 of which remain standing), since each stands a whopping 62 feet tall and 7.5 feet in diameter.
Hagar Qim (Malta)
Located on the Mediterranean island of Malta, the UNESCO World Heritage Site of Hagar Qim is one of seven prehistoric temples in Malta. It is believed to date to between 3800 BCE and 2200 BCE. The site’s name translates to “standing stones,” and one of the largest weighs in at more than 20 tons and measures nearly 23 feet in height. The site was first excavated in 1839 and consists of a series of rooms lined by these megaliths. Parts of the chamber align with the sunrise and sunset of the summer solstice. This and the other temples on the island all appear to have been built in the same period, which has left archaeologists puzzled — there is little evidence of any civilization capable of such building feats on the islands at that time.
Göbekli Tepe (Turkey)
Could a set of ruins in southeastern Turkey be remnants of the world’s first temple? That’s one of the key questions archaeologists ponder as they explore Göbleki Tepe, a series of huge stone pillars that are some 6,000 years older than Stonehenge. Now a UNESCO World Heritage Site, the landmark was ignored for centuries, dismissed as little more than a cemetery. In the mid-1990s, excavations began and experts soon realized it was a treasure trove of history. The pillars weigh as much as 10 tons each and create massive stone circles. Radar surveys of the area indicate a number of additional circles are still buried underground. Göbleki Tepe is older than writing and older than agriculture. But who were the Neolithic people who built this, and how and why did they do it?
Yonaguni Monument (Japan)
Experts are divided as to whether the underwater rocks near Japan’s Yonaguni Island are a human-made structure or naturally occurring. In the 1980s, divers discovered what appears to be a rectangular monument, measuring 165 feet long and 65 feet wide. Some scholars believe that it is the remains of a pyramid, perhaps from a long-lost submerged city belonging to an ancient civilization. Meanwhile, others insist the rocks have been shaped by millennia of the ocean’s currents. Similarly, while some argue that markings on the rock’s surface are proof of ancient human involvement, others say they are simply scratches. For the time being, the Japanese government seems to agree with the latter and does not recognize the Yonaguni Monument as culturally significant.
Great Zimbabwe Ruins (Zimbabwe)
The Great Zimbabwe Ruins are the largest ruins in sub-Saharan Africa. This medieval city was once a trading hub and possibly the capital of the Queen of Sheba’s realm. The remains consist of the Great Enclosure (perhaps a royal residence), the Hill Complex (possibly the religious heart of the city), and the Valley Ruins (houses which suggest the city once had a population of 20,000 people). In total, the Great Zimbabwe Ruins extend across an area of 200 acres. The city is thought to have been abandoned in the 15th century, for reasons scientists aren’t sure of.
The Maya people of what is now Mexico were incredibly advanced when it came to writing, building, and knowledge of astronomy. Yet scientists still know little about other parts of their culture. By the time Spanish conquistadors arrived from Europe, the Maya civilization had already fallen, and historians still debate the cause. Some of the finest Maya ruins are at Palenque, in the Mexican state of Chiapas, an elaborate complex that includes a palace and several temples. Thought to have been constructed between 500 and 700 CE, it features plaster carvings and decorations that are still remarkably well-preserved. The city at Palenque is a marvel of design but remains shrouded in mystery since we may never know why it was abandoned around 900 CE.
No list of mysterious sites would be complete without the Neolithic monument at Stonehenge, which continues to mystify visitors from all over the world. The enormous stones are estimated to have been placed between 2500 BCE and 2200 BCE. Hundreds of even older burial mounds have also been uncovered in the surrounding area. Some of the stones come from several hundred miles away in Wales, leading archaeologists to speculate how they were transported. Others are from nearer parts of Wiltshire. What was Stonehenge’s purpose? Many believe it was a spiritual site, and people still flock to it as the sun rises on the summer solstice, when sunlight rises above the Heel stone at Stonehenge and falls directly onto the middle of the circle.