The world is filled with ancient monuments built by  master craftsmen in order to honor everything from kings and presidents  to religious figures. And although most of these landmarks have been  carefully studied and researched by scientists and historians, some are  simply so old, incomplete, or obscure that we still don’t know very much  about why they were built or what purpose they served. The following  are 
10 world landmarks that, whether by intention or simply due to the passage of time, continue to baffle the people who study them. 
  10. The Cahokia Mounds
   
    
  Cahokia  is the name given to an Indian settlement that exists outside of  Collinsville, Illinois. Archeologists estimate that the city was founded  sometime around 650 AD, and its complex network of burial grounds and  sophisticated landscaping prove that it was once a thriving community.  It has been estimated that at its peak the city was home to as many as  40,000 people, which would have made it the most populous settlement in America  prior to the arrival of the Europeans. The most notable aspect of  Cahokia today are the 80 mounds of earth, some as high as 100 feet,  which dot the 2,200-acre site. These helped create a network of plazas  throughout the city, and it is believed that important buildings, like  the home of the settlement’s chief, were built on top of them. The site  also features a series of wooden posts that archeologists have dubbed  “woodhenge.” The posts are said to mark the solstices and equinoxes, and  supposedly figured prominently in the community’s astronomical  mythology.
  The Mystery  
Although scientists are constantly discovering new information about the Cohokia  community, the biggest mystery that remains is which modern Indian  tribe is descended from the residents of the ancient city, as well as  just what it was that caused them to abandon their settlement.
  9. Newgrange
   
    
  Considered  to be the oldest and most famous prehistoric site in all of Ireland,  Newgrange is a tomb that was built from earth, wood, clay, and stone  around 3100 BC, some 1000 years before the construction of the pyramids in Egypt.  It consists of a long passage that leads to a cross-shaped chamber that  was apparently used as a tomb, as it contains stone basins filled with  cremated remains. The most unique feature of Newgrange  is its careful and sturdy design, which has helped the structure remain  completely waterproof to this day. Most amazing of all, the entrance to  the tomb was positioned relative to the sun in such a way that on the  winter solstice, the shortest day of the year, the rays from the sun are  channeled through the opening and down the nearly 60 foot passageway,  where they illuminate the floor of the monument’s central room.
  The Mystery  
Archeologists know Newgrange  was used as a tomb, but why and for who still remains a mystery. The  painstaking design needed to guarantee that the yearly solstice event  occurs suggests that the site was held in high regard, but other than  the obvious hypothesis that the sun featured prominently in the  mythology of the builders, scientists are at a loss to describe the true  reason for Newgrange’s construction.
  
  8. The Yonaguni Monument
   
    
  Of all the famous monuments in Japan,  perhaps none is more perplexing than Yonaguni, an underwater rock  formation that lies off the coast of the Ryuku Islands. It was  discovered in 1987 by a group of divers who were there to observe  Hammerhead sharks, and it immediately sparked a huge amount of debate in  the Japanese scientific community. The monument is made up of a series  of striking rock formations including massive platforms, carved steps,  and huge stone pillars that lie at depths of 5-40 meters. There is a  triangular formation that has become known as “the turtle” for its  unique shape, as well as a long, straight wall that borders one of the  larger platforms. The currents in the area are known for being  particularly treacherous, but this has not stopped the Yonaguni monument  from becoming one of the most popular diving locations in all of Japan.
  The Mystery  
The ongoing debate surrounding Yonaguni  centers on one key subject: is the monument a natural phenomenon, or is  it man-made? Scientists have long argued that millennia of strong  currents and erosion have carved the formations out of the ocean floor,  and they point to the fact that the monument is all one piece of solid  rock as proof that it was not assembled by a builder. Others, though,  point to the many straight edges, square corners and 90-degree angles of  the formation as proof that it’s artificial. They often cite one  formation in particular, a section of rock that resembles a crude  carving of a human face, as evidence. If they are right, then an even  more interesting mystery presents itself: who constructed the Yonaguni  Monument, and for what purpose?
  7. The Nazca Lines
   
    
  The Nazca lines are a series of designs and pictographs carved into the ground in the Nazca Desert, a dry plateau located in Peru.  They cover an area of some 50 miles, and were supposedly created  between 200 BC and 700 AD by the Nazca Indians, who designed them by  scraping away the copper colored rocks of the desert floor to expose the  lighter-colored earth beneath. The lines have managed to remain intact  for hundreds of years thanks to the region’s arid climate, which sees it  receive little rain or wind throughout the year. Some of the lines span  distances of 600 feet, and they depict everything from simple designs  and shapes to characterizations of plants, insects, and animals.
  The Mystery  
Scientists know who made the Nazca Lines  and how they did it, but they still don’t know why. The most popular  and reasonable hypothesis is that the lines must have figured in the  Nazca people’s religious beliefs, and that they made the designs as  offerings to the gods, who would’ve been able to see them from the  heavens. Still, other scientists argue that the lines are evidence of  massive looms that the Nazcas used to make textiles, and one  investigator has even made the preposterous claim that they are the  remnants of ancient airfields used by a vanished, technologically  advanced society.
  6. Goseck Circle
   
    
  One of the most mysterious landmarks in Germany is the Goseck Circle,  a monument made out of earth, gravel, and wooden palisades that is  regarded as the earliest example of a primitive “solar observatory.” The  circle consists of a series of circular ditches surrounded by palisade  walls (which have since been reconstructed) that house a raised mound of  dirt in the center. The palisades have three openings, or gates, that  point southeast, southwest, and north. It is believed that the monument  was built around 4900 BC by Neolithic peoples, and that the three  openings correspond to the direction from which the sun rises on the  winter solstice.
  The Mystery  
The  monument’s careful construction has led many scientists to believe that  the Goseck Circle was built to serve as some kind of primitive solar or  lunar calendar, but its exact use is still a source of debate. Evidence  has shown that a so-called “solar cult” was widespread in ancient  Europe. This has led to speculation that the Circle was used in some  kind of ritual, perhaps even in conjunction with human sacrifice. This  hypothesis has yet to be proven, but archeologists have uncovered  several human bones, including a headless skeleton, just outside the  palisade walls. 
  
  5. Sacsayhuaman
   
    
  Not far from the famous Inca city of Machu Picchu  lies Sacsayhuaman, a strange embankment of stone walls located just  outside of Cuzco. The series of three walls was assembled from massive  200-ton blocks of rock and limestone, and they are arranged in a zigzag  pattern along the hillside. The longest is roughly 1000 feet in length  and each stands some fifteen feet tall. The monument is in astonishingly  good condition for its age, especially considering the region’s  propensity for earthquakes, but the tops of the walls are somewhat  demolished, as the monument was plundered by the Spanish to build  churches in Cuzco. The area surrounding the monument has been found to  be the source of several underground catacombs called chincanas, which  were supposedly used as connecting passageways to other Inca structures  in the area.
  The Mystery  
Most scientists  agree that Sacsayhuaman served as a kind of fortress of barrier wall,  but this has been disputed. The strange shape and angles of the wall  have led some speculate that it may have had a more symbolic function,  one example being that the wall, when seen next to Cuzco from above,  forms the shape of the head of a Cougar. Even more mysterious than the  monument’s use, though, are the methods that were used in its  construction. Like most Inca stone works, Sacsayhuaman was built with  large stones that fit together so perfectly that not even a sheet of  paper can be placed in the gaps between them. Just how the Incas managed  such expert placements, or, for that matter, how they managed to  transport and lift the heavy hunks of stone, is still not fully known. 
  
  4. The Easter Island Moai
   
    
  One  of the most iconic series of monuments in the Pacific islands is the  Moai, a group of huge statues of exaggerated human figures that are  found only on the small, isolated island of Rapa Nui, or Easter Island.  The Moai were carved sometime between 1250 and 1500 AD by the island’s  earliest inhabitants, and are believed to depict the people’s ancestors,  who in their culture were held in the same regard as deities. The Moai  were chiseled and carved from tuff, a volcanic rock that is prevalent on  the island, and they all feature the same characteristics of an  oversized head, broad nose, and a mysterious, indecipherable facial  expression. Scientists have determined that as many as 887 of the  statues were originally carved, but years of infighting among the  island’s clans led to many being destroyed. Today, only 394 are still  standing, the largest of which is 30 feet tall and weighs over 70 tons.
  The Mystery  
While  there is a fairly solid consensus on why the Moai were erected, how the  islanders did it is still up for debate. The average Moai weighs  several tons, and for years scientists were at a loss to describe how  the monuments were transported from Rano Raraku, where most of them were  constructed, to their various locations around the island. In recent  years, the most popular theory is that the builders used wooden sleds  and log rollers to move the Moai, an answer that would also explain how  the once verdant island became almost totally barren due to  deforestation.
  
  3. The Georgia Guidestones
   
    
  While most of the mysterious monuments  on this list only became that way as centuries passed, the Georgia  Guidestones, also known as American Stonehenge, are one landmark that  was always intended to be an enigma. The monument, which consists of  four monolithic slabs of granite that support a single capstone, was  commissioned in 1979 by a man who went by the pseudonym of R.C.  Christian. A local mason carefully crafted it so that one slot in the  stones is aligned with the sun on the solstices and equinoxes, and one  small hole is always pointed in the direction of the North Star. Most  interesting, though, are the inscriptions on the slabs, which an  accompanying plaque describes as “the guidestones to an Age of Reason.”  In eight different languages, the slabs offer a strange ten-point plan  to ensure peace on Earth that includes vague proclamations like “prize  truth–beauty–love–seeking harmony with the infinite,” to very specific  commands like “maintain humanity under 500,000,000 in perpetual balance  with nature.” Comments like this one have made the Guidestones one of  the most controversial landmarks in the United States, and they have  long been protested and even vandalized by groups that would like to see  them demolished.
  
  The Mystery  
For  all their controversy, very little is known about who built the  Guidestones or what their true purpose is. R.C. Christian claimed he  represented an independent organization when he commissioned the  landmark, but neither he nor his group has spoken up since its  construction. Since the monument was built during the height of the Cold  War, one popular theory about the group’s intentions is that the  Guidestones were to serve as a primer for how to rebuild society in the  aftermath of a nuclear holocaust. 
  
  2. The Great Sphinx of Giza
   
    
  Sphinxes are  massive stone statues that depict the body of a reclining lion with the  head and face of a human. The figures are found all over the world in  different forms, but they are most commonly linked with Egypt, which  features the most famous example in the form of the Great Sphinx of Giza.  Incredibly, the statue is carved out of one monolithic piece of rock,  and at 240 feet long, 20 feet wide, and 66 feet tall, it is considered  to be the biggest monument of its kind in the world. Historians largely  accept the function of the Sphinx to have been that of a symbolic  guardian, since the statues were strategically placed around important  structures like temples, tombs, and pyramids. The Great Sphinx of Giza  appears to be no different. It stands adjacent to the pyramid of the  pharaoh Khafra, and most archeologists believe that it is his face that  is depicted on that of the statue. 
  The Mystery  
Despite  its reputation as one of the most famous monuments of antiquity, there  is still very little known about the Great Sphinx of Giza.  Egyptologists might have a small understanding of why the statue was  built, but when, how, and by who is still shrouded in mystery. The  pharaoh Khafra is the main suspect, which would date the structure back  to around 2500 BC, but other scientists have argued that evidence of  water erosion of the statue suggests that it is much older and perhaps  even predated the dynastic era of the Egyptians.  This theory has few modern adherents, but if true it would mean the  Great Sphinx of Giza is even more mysterious than previously believed. 
  
  1. Stonehenge
   
    
  Of all the world’s famous monuments, none has gained as much of a reputation for pure, simple mystery as Stonehenge. Stonehenge has been inspiring debate among scholars, scientists, and historians since the Middle Ages. Located in the English countryside,  the landmark is believed to date back to 2500 BC, and consists of  several mammoth pieces of rock arranged and piled on top of one another  in what appears at first to be a random design. The site is surrounded  by a small, circular ditch, and is flanked by burial mounds on all  sides. Although the rock formations that still remain are undoubtedly  impressive, it is thought that the modern version of Stonehenge  is only a small remnant of a much larger monument that was damaged with  the passing of time, and it is largely believed that the building  process was so extensive that it could have lasted on and off for  anywhere from 1500 to 7000 years.
  The Mystery  
Stonehenge  has become renowned for puzzling even the most brilliant researchers,  and over the years the many gaps in the history of its construction, the  nature of its use, and the true identity of its builders have become  known as “
The Mystery of Stonehenge.”  The Neolithic people who built the monument left behind no written  records, so scientists can only base their theories on the meager  evidence that exists at the site. This has led to wild speculation that  the monument was left by aliens, or that it was built by some eons-old  society of technologically advanced super-humans. All craziness aside,  the most common explanation remains that Stonehenge served as some kind  of graveyard monument that played a role in the builders’ version of the  afterlife, a claim that is backed up by its proximity to several  hundred burial mounds. Yet another theory suggests that the site was a  place for spiritual healing and the worship of long dead ancestors.