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 11 such monuments that remain a mystery.
Original photo by Stephanie LeBlanc/ Unsplash
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 — upright stones arranged by humans. There is 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 who were petrified by Pope Cornelius. Local folklore, meanwhile, says 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 people 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 sometime 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, and some even think they may be remnants of the lost city of Atlantis. Some of the spheres were even detonated in the hope of finding gold inside.
Temple of Bacchus (Lebanon)
The Baalbek temple complex in northeast Lebanon is one of the most intriguing Roman ruins on the planet. Its centerpiece is 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. Most 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 staggering to think that humans without heavy machinery could hoist the 42 Corinthian columns (19 of which remain standing) of the colonnade, since each stands 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 and 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, measuring 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 is known worldwide and continues to mystify visitors. 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.
For most of recorded human history, time has been carved up into various numbers of days, months, and years. Some ancient cultures relied on the moon to note the passage of days, and this ancient tradition still impacts the way we talk about the calendar (the words “moon” and “month” are actually related). Eventually, mathematicians and astronomers encouraged counting the days using another prominent feature of Earth’s sky — the sun.
Over the course of a few millennia, the calendar has been shaped and rearranged to fit fleeting political whims, religious observances, bureaucratic challenges, and bizarre superstitions. The story of the calendar is the story of humanity, and the answers to these questions show why.
Why Are There 12 Months?
At its start in the eighth century BCE, Rome used a 10-month calendar traditionally believed to be created by its legendary wolf-suckling founder, Romulus. This was a lunar calendar: The beginning of a month, or a new moon, was called the “kalends,” while a waxing half-moon around the seventh of the month was called the “nones,” and a full moon around the 15th of the month was called the “ides.” In this calendar, the year started with March, ended in December, and only added up to about 304 days. So what happened to the 60 or so days between December and March? Well, nothing — Romans just waited for the first new moon before the spring equinox to start the new year, meaning that much of the winter passed in a period without a calendar.
This system, understandably, didn’t work well, and was soon reformed by Rome’s second king, Numa Pompilius, around 713 BCE. Pompilius added additional months — now called January and February — to the end of the year, creating a 12-month calendar (they eventually moved to the front of the year by 450 BCE). The months totaled 354 days, but because of a Roman superstition around even numbers, an extra day was added to January. Since 355 days is still out of sync with the solar year and thus the seasons and celestial events, the king then added extra days, called intercalation, to the latter part of February in certain years. This made the Roman calendar’s average length 366.25 days long — still off, but much better than Romulus' temporal train wreck.
Pompilius’ creation was eventually undermined by Roman pontifices, or priests, who wielded intercalation like a political cudgel — extending the rule of favored politicians while curtailing the term limits of enemies. After 700 years, the Roman calendar was a mess, and the powerful general and statesman Julius Caesar decided to fix it. Following consultation with Rome’s greatest mathematicians and astronomers, he implemented the Julian calendar in 45 BCE. Influenced by the 365-day Egyptian calendar and the mathematics of the Ancient Greeks, this calendar discarded Pompilius’ even number superstition and added extra days equaling 365. But the most notable advancement of Caesar’s calendar was that it embraced the sun as the basis of the calendar rather than the moon. Finally, after 700 long, horribly mismanaged years, the calendar was divided into our modern 12 months.
Where Do the Names of the Months Come From?
The short answer is Rome, but the long answer is much more interesting. Remember Romulus’ 10-month calendar? Well, September, October, November, and December simply mean “seventh month,” “eighth month,” “ninth month,” and “tenth month” in Latin, respectively. But these names no longer made sense after the later additions of January, named after the Roman god Janus, and February, named after the Roman purification festival Februa. As for the rest of the months, March is named for the Roman god Mars, April after the Greek goddess Aphrodite (though there’s some debate about whether it might be based on the Latin word aperio, which means “I open” in relation to spring flowers), May after the Greek deity Maia, and June in honor of the powerful Roman goddess Juno.
The names of the last two months come from a few powerful Romans who got a little full of themselves. In 44 BCE, the month Quintilis (which means “fifth” in Latin) was changed to July in honor of Julius Caesar. His heir, Augustus, received the same honor in 8 BCE, when Sextilis (you guessed it, meaning “sixth” in Latin) was changed to August.
Why Is February the Shortest Month of the Year?
February has fewer days because of the superstitions of ancient Rome. In the late eighth century BCE, Romans — including their king Numa Pompilius — held a superstition that even numbers were somehow unlucky. Although he created a version of a 12-month calendar, Pompilius realized there was no mathematical way for every month to have an odd number of days and for the total number of days in the year to also be odd. So while the other months were either 29 or 31 days long, February became the unlucky month to have only 28 days, making Pompilius’ calendar the apparently-less-scary number of 355.
In 45 BCE, Caesar — disregarding Pompilius’ fear of even numbers — added days to a number of other months, but not February. Some experts believe Caesar didn’t want to disrupt the important festivals that took place in that month and so he just let it be. But with the introduction of the Julian calendar, February did receive a consolation prize in the form of an additional day every four years. Speaking of which …
Why Do We Need a Leap Day?
A year isn’t 365 days, it’s actually 365.24219 days. Because of our planet’s frustratingly imperfect solar orbit, calendars need small adjustments as the years pass to keep in alignment with equinoxes and solstices. Ancient astronomers and mathematicians figured that waiting four years and then adding a day made the most sense. In 45 BCE, Julius Caesar introduced the modern leap year, which added an extra day in February every four years (though originally that extra day was added between the 23rd and the 24th). This moved the calendar closer to solar reality at 365.25 days. Close, but not close enough — which is where the pope comes in.
Who Made the Modern Calendar?
In 1582, Pope Gregory XIII had a problem. As head of the Catholic Church, he realized that Easter — his religion’s holiest day — had drifted 10 days off in relation to the spring equinox, which is supposed to be used to calculate Easter day. That’s because Caesar’s small mathematical error had grown exponentially larger when stretched across 1,600 years. Gregory XIII needed a very slight adjustment to the calendar, just enough to nudge it closer to that magical 365.24219 number. First, Gregory XIII lopped 10 days off the calendar to set things straight, then tweaked the leap year. Now, whenever a new century began that wasn’t divisible by 400 (i.e. 1700, 1800, 1900), no extra day was added. This edged things just enough in the right direction that this new calendar, named the Gregorian calendar, was now 365.2425 days long — close enough. Catholic nations adopted this new calendar immediately, but the Protestant British Empire, along with its American colonies, didn’t sign on until 1752. Today, the Gregorian calendar is used in nearly every country.
When Did We Start Using B.C. and A.D.?
Before the invention of A.D. (“anno domini,” which means “in the year of our Lord”) and B.C. (“before Christ”), years were often tracked by the reigns of pharaohs, kings, and emperors. In a way, B.C. and A.D. still reflect this system but focus on just one moment — the birth of Jesus. It’s difficult to trace the exact origins of this system, but one of the earliest recorded uses of “anno domini” occurs in 525 with the work of Dionysius Exiguus, a monk who was trying to determine what days Easter would fall in future years. Crucially, he started his tables with the year 532, stating that this year was “from the incarnation of our Lord Jesus Christ.”
The conception of “B.C.” is slightly murkier. Some believe the Venerable Bede, the famous medieval English historian, was the first to use it, or at least greatly popularized it in his 731 work, Ecclesiastical History of the English People. Others point to a 1627 work by a French Jesuit who used “ante Christum” to describe the pre-Jesus years. The terminology became more widespread during the reign of the Holy Roman Emperor Charlemagne, who used it as a standard form of dating across Europe in the ninth century.
Within the last few decades, more publications and organizations have opted to strip the years of their religious connotation, preferring BCE (Before the Common Era) and CE (Common Era) over the traditional B.C./A.D. system, although the move is not without some controversy. But this subtle change in phrasing doesn’t alter the fact that the world still counts the years in accordance with the birth of Jesus.
Why Is a Week Seven Days?
The seven-day week is a timekeeping oddity. Unlike days, months, and years, the week doesn’t align with any celestial reality, and it doesn’t divide elegantly into existing periods of time. For example, there aren't 52 weeks in an average year — there are 52.1428571429. So how did this happen? Babylonians, the ancient superpower of Mesopotamia, put a lot of stock in the number seven thanks to the seven observable celestial bodies in the night sky — the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn. This formed the seven-day week, which was adopted by the Jewish people, who were captives of the Babylonians in the sixth century BCE. Eventually, it spread to ancient Greece and elsewhere thanks to the battle-happy Macedonian Alexander the Great. Efforts have been made throughout history to reform the seven-day week, but this oddball unit of time has become ingrained in many religions, including Judaism, Christianity, and Islam, rendering any sort of tweak pretty unlikely.