Millennia from now, future archaeologists will probably wonder why France decided to build a big metal tower in the center of Paris. They'll come upon the ruins of Las Vegas and puzzle over why 20th century humans built a glitzy metropolis in the middle of nowhere. They'll find the ruins of the Brooklyn Bridge and marvel over how it could have been constructed using only steam-powered machines.
For modern archaeologists, the ancient world continues to hold many secrets. Civilizations disappear, and thousands of years later, researchers are left to pick through the ivy-covered ruins of their cities.
The following is a list of five awe-inspiring sights whose codes haven't been cracked yet. Who built them? Why did they build them? Did humans even build them at all?
Early Europeans explorers described Easter Island as a wasteland. With no trees and no shrubs, the isolated Pacific island was nothing more than a New York-sized chunk of grassland. But peppering this seemingly dead landscape were almost 1,000 moai: Huge monolithic human statues weighing up to 86 tons.
Without materials for making rope or lumber, visitors wondered, how could the early inhabitants of Easter have mounted these enormous sculptures into place? Gradually, scientists unearthed the chilling details underlying the Island's statue-obsession. Five hundred years ago, they found, Easter Island had been a tropical paradise hosting lush forests and as many as 20,000 residents.
According to author Jared Diamond, it was the monuments themselves that drove Easter Island to its present ruin [source: Diamond]. Caught in a moai-building frenzy, Easter Islanders stripped the island's forests bare for materials. Without proper habitat, birds went extinct. And without lumber for making canoes, Islanders could no longer hunt for porpoises -- one of their main sources of food. Inevitably, the once-thriving residents of Easter Island were forced to resort to cannibalism.
Scientists have a pretty good idea why Easter Islanders built the moai. They were status symbols, pure and simple: Easter's rival groups were locked in a fatal race to one-up their competitors. But what continues to amaze researchers is how Easter Islanders were seemingly powerless to stop the destruction of their own home.
Believed to be the center of a vast empire, Teotihuacan was one of the world's largest cities in 600 AD, holding as many as 200,000 inhabitants [source: Metropolitan Museum]. Teotihuacan's lasting centerpiece is a massive boulevard known as the Avenue of the Dead. Stretching for 1.6 miles (2.5 kilometers), the boulevard is flanked by massive platforms, muraled walls, sunken plazas and two pyramids large enough to rival the Great Pyramids of Egypt.
To this day, nobody knows who built Teotihuacan, or what became of its inhabitants. The ancient Aztecs, who lived among Teotihuacan's ruins, were oblivious to the city's original architects -- although they worshiped them for their skills (the word Teotihuacan is an Aztec word meaning "place where men became gods") [source: Sugiyama].
With no royal tombs or artwork depicting the city's leaders, Teotihuacan offers few clues for archaeologists. However, they've uncovered some evidence of the city's violent end. At some point, Teotihuacan began embracing a culture of increasing militarism, possibly as a way to strong-arm nearby groups into paying tribute to the ballooning metropolis. Sometime around the 7th century, brutal conflict erupted, portions of the city were repeatedly sacked and burned, and the ancient city was quickly abandoned.
Machu Picchu isn't a "lost city," as many tourist brochures would have you believe. It was simply a summer hangout for the Inca king. Undiscovered by Spanish conquistadors, it wasn't until 1911 that American explorer Hiram Bingham broadcast Machu Picchu's secrets to the world. The site is now Peru's most-visited tourist attraction.
Completed around 1450, Machu Picchu is an undisturbed masterwork of Inca craftmanship nestled high in the remote Andes. Supporting as many as 750 residents during the summer months, the seasonal retreat is a carefully-planned patchwork of homes and temples held up by a mammoth network of underground walls. Acres of terraced farms notched into the Andean mountainsides provided the settlement with food.
Most amazingly, the entire city is constructed from interlocking walls of smooth, polished stones. Without using mortar, the Inca fit the stones together like an immense jigsaw puzzle. Their work was so precise that, even after centuries of earthquakes, in many places it's still impossible to slip a piece of paper between the seams of two Machu Picchu stones.
The mystery of Machu Picchu is how the Inca were able to move such large stones to such a remote location. Although the rock was quarried locally, workers would still have needed to hoist 20-ton stones up steep mountain cliffs -- an especially grueling task when you consider that the Inca didn't use wheels. Ultimately, the coming of the Spanish conquistadors was so devastating to the Inca that within a generation after the arrival of Europeans, nobody was left alive to recall the secrets of their mountaintop city.
While clearing the Costa Rican jungle for banana plantations in 1940, employees of the United Fruit Company began uncovering large stone spheres buried in the forest floor.
Almost immediately, the mysterious spheres became prized lawn ornaments, ending up on the front yards of government buildings and fruit company executives throughout Costa Rica. Many spheres were broken in transit, and others were purposely dynamited by treasure hunters.
It was a sorry fate for one of the region's greatest archaeological treasures. Hundreds of spheres, ranging in size from a basketball to a compact car, have been unearthed in the last 70 years. Carved by the ancestors of Costa Rica's indigenous people, the near-perfect spheres would have needed to be shaped using only other stones, since they didn't have access to metal tools. Sphere-carving was much more than a passing fad: Archaeologists estimate that native Costa Ricans were carving spheres from as early as 600 AD to as late as 1500 AD [source: Hoopes].
Archaeologists remain stumped over the purpose of the spheres. The people who made them had no written records, and their culture was decimated soon after the Spanish conquest. Much like the Easter Island moai, one theory assumes that the spheres were simply status symbols. The stones also might have once been arranged into massive patterns that had astronomical significance. Since almost every sphere has been moved from its original location, however, researchers are sceptical that the true meaning of the spheres will ever be discovered.
It could be a random pile of rocks, or it could be the Asian Atlantis. Located just off the shores of Yonaguni, one of Japan's most southerly islands, the Yonaguni Monument is a vast collection of stone structures believed by some to be the sunken remnants of an ancient civilization. At first glance, Yonaguni's human origins seems obvious: The site is composed of right-angled stone walls topped by a massive stone pyramid. Japanese marine geologist Masaaki Kimura, who has studied Yonaguni for 15 years, maintains the sprawling site once held a stadium, five temples, a triumphal arch and a castle [source: Ryall]. In certain spots, Kimura says, stones are nicked by what appear to be quarry marks.
Skeptics maintain that the monument's resemblance to an ancient city is merely a coincidence. Sandstone naturally breaks along straight lines -- especially in earthquake zones -- and could have easily formed the monument. To look at maps of the site, Yonaguni certainly doesn't seem like a very welcoming city. Some of the monument's high terraces appear to be joined by "staircases" with steps that are up to a meter high. Peculiar for an archaeological site, the Monument has also been found to contain no tools or pottery.
Boston University professor Robert Schoch (who also theorizes that Easter Island was built by ancient giants) thinks that Yonaguni may be a mixture of both natural and manmade structures. Before the monument was buried by a massive tsunami, ancient humans may have lived among its formations, says Schoch. And just as prehistoric humans decorated the caves of France, the residents of the Yonaguni Monument "spruced up" the place with some carvings and tombs [source: Schoch]. So far, the Japanese government isn't buying any of this. More than 20 years after it was discovered, state officials have yet to send an official expedition to the site.
HowStuffWorks looks at why the Taj Mahal is in such poor shape, why it's so hard to repair and how this problem happens in other places like Italy.
- Travel the World: The Egyptian Pyramids
- Travel the World: Machu Picchu
- 10 Most Dangerous Places You Should Definitely Visit
- Top 5 Unsolved Brain Mysteries
- How the Bermuda Triangle Works
- Are there undiscovered people?
- Did Egyptians really build the Sphinx?
- Curiosity Project: Archeological Finds of the 21st Century Pictures
More Great Links
- BBC News. "The stone spheres of Costa Rice." March 29, 2010. (Oct. 3, 2010) http://news.bbc.co.uk/2/hi/science/nature/8593717.stm
- Collyns, Dan. "Machu Picchu Ruin 'Found Earlier.'" June 6, 2008. (Oct. 3, 2010) http://news.bbc.co.uk/2/hi/americas/7439397.stm
- Diamond, Jared. "Easter's End." August 1995. (Oct. 3, 2010) http://discovermagazine.com/1995/aug/eastersend543
- Dunning, Brian. "Yonaguni Monument: The Japanese Atlantis." Aug. 24, 2010. (Oct. 3, 2010) http://skeptoid.com/episodes/4220
- Hunt, Terry. "Rethinking the Fall of Easter Island." October 2006. (Oct. 3, 2010) http://www.americanscientist.org/issues/feature/rethinking-the-fall-of-easter-island/1
- Leff, Alex. "Costa Rica's Mysterious Stone Spheres." March 28, 2010. (Oct. 3, 2010) http://www.globalpost.com/dispatch/costa-rica/100324/stone-spheres-unesco
- McMahan, Bucky. "Secrets of Yonaguni." Nov. 5, 2006. (Oct. 3, 2010) http://www.scubadiving.com/travel/2006/11/secrets-of-yonaguni
- Metropolitan Museum of Art. "Teotihuacan." (Oct. 3, 2010) http://www.metmuseum.org/toah/hd/teot/hd_teot.htm
- Ryall, Julian. "Japan's Ancient Underwater Pyramid Mystifies Scholars." Sept. 19, 2007. (Oct. 3, 2010) http://news.nationalgeographic.com/news/2007/09/070919-sunken-city.html
- Schoch, Robert. "An Enigmatic Ancient Underwater Structure off the Coast of Yonaguni Island, Japan." 1999. (Oct. 3, 2010) http://circulartimes.org/Enigmatic%20Yonaguni%20Underwater%20RMS%20CT.htm
- Schoch, Robert. "Yonaguni." (Oct. 10, 2010) http://www.robertschoch.com/yonagunicontent.html
- Science Daily. "Easter Island's Controversial Collapse: More to the Story than Deforestation?" February 18, 2009. (Oct. 3, 2010) http://www.sciencedaily.com/releases/2009/02/090218095435.htm
- Science Daily. "Mysterious Stone Spheres in Costa Rice Investigated." March 23, 2010. (Oct. 3, 2010) http://www.sciencedaily.com/releases/2009/02/090218095435.htm
- Science Daily. "Sacrificial Burial Deepens Mystery at Teotihuacan, But Confirms the City's Militarism." December 9, 2004. (Oct. 3, 2010) http://www.sciencedaily.com/releases/2004/12/041203084345.htm
- Stevenson, Mark. "Teotihuacan Tunnels Thrill Archaeologists." Aug. 4, 2010. (Oct. 3, 2010) http://www.iol.co.za/teotihuacan-tunnel-thrills-archaeologists-1.671893
- Sugiyama, Saburo. "Teotihuacan." Aug. 20, 2001. (Oct. 3, 2010) http://archaeology.asu.edu/teo/intro/intrteo.htm
- Thompson, Nigel. "Machu Picchu, Peru: Ancient Andes city is Picchu-perfect." Oct. 2, 2010. (Oct. 3, 2010) http://www.mirror.co.uk/advice/travel/2010/10/02/picchu-perfect-115875-22602064/
- United Nations Environment Programme. "Historic Sanctuary of Machu Picchu." (Oct. 3, 2010) http://www.unep-wcmc.org/sites/wh/pdf/Machu%20Picchu.pdf