1. Maya

Ancient Civilizations that Mysteriously Collapsed: Maya
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A section of the Mayan Troano Codex, one of three surviving pre-Columbian Maya books.

Arguably the New World’s most advanced pre-Columbian civilization, the Maya carved large stone cities into the jungles of southern Mexico and Central America, complete with elaborate plazas, palaces, pyramid-temples and ball courts. Known for their hieroglyphic writing, as well as their calendar-making, mathematics, astronomy and architecture skills, the Maya reached the peak of their influence during the so-called Classic Period, from around A.D. 250 to A.D. 900. But at the end of the Classic Period, in one of history’s great enigmas, the populace suddenly deposed its kings, abandoned the cities and ceased with technological innovation.

Dozens of theories have been put forth to explain what happened. Some historians, for instance, point to a major drought, exacerbated by deforestation and soil erosion, as the impetus for the societal collapse, while others put the blame on a disease epidemic, a peasant revolt against an increasingly corrupt ruling class, constant warfare among the various city-states, a breakdown of trade routes or some combination thereof. Though dispersed, the Maya never disappeared. Millions of their Mayan-speaking descendants continue to inhabit the region to this day.

READ MORE: Why the Maya Abandoned Their Cities 

2. Indus

Ancient Civilizations that Mysteriously Collapsed: Indus
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A sculpture, possibly of a priest, from the ancient city of Mohenjo-Daro, now in the Karachi Museum in Pakistan.

The Indus began building settlements in present-day India and Pakistan as early as 8,000 years ago, making them one of the earliest civilizations. By the third millennium B.C., they occupied over 386,000 square miles of territory—much more than their better-known contemporaries in Egypt and Mesopotamia—and accounted for an estimated 10 percent of the world’s population. They also developed a writing script that’s still yet to be deciphered, and their cities contained sanitation systems that remained unequaled until Roman times.

Around 1900 B.C., however, the Indus, also known as the Indus Valley or Harappa civilization, went into freefall. The population abandoned the cities and purportedly migrated to the southeast. Originally, scholars believed that an Aryan invasion from the north brought about the Indus collapse, but that theory is no longer in vogue. Recent research instead suggests that the monsoon cycle essentially stopped for two centuries, making agriculture nearly impossible. Other factors, such as earthquakes or outbreaks of malaria or cholera, may have also played a role.

3. Anasazi

Ancient Civilizations that Mysteriously Collapsed: Anasazi
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Cliff Palace, located in present day Mesa Verde National Park, built by ancestral Puebloans.

In the dry Four Corners region of the present-day United States, the Anasazi built spectacular stone dwellings into the sides of cliffs during the 12th and 13th centuries, some of which contained hundreds of rooms. No other U.S. building would be taller until the construction of the first skyscrapers in the 1880s. Yet the cliff dwellings did not remain occupied for long, and the end apparently wasn’t pretty.

Researchers have uncovered signs of massacres and cannibalism, as well as evidence of deforestation, water management problems and a crippling long-term drought that many believe precipitated the slide into violence. Religious and political upheaval, akin to what Europe faced following the Protestant Reformation, may have added to the chaos, which ultimately forced the Anasazi to abandon their homeland by A.D. 1300 and flee south. Their modern-day descendants include the Hopi and Zuni peoples, some of whom consider the term Anasazi offensive, preferring instead to say “ancestral (or ancient) Puebloans.”

READ MORE: Ancient Native Americans Once Thrived in Bustling Urban Centers

4. Cahokia

READ MORE: Ancient Native Americans Once Thrived in Bustling Urban Centers

Thanks to the spread of corn cultivation from Mexico, indigenous villages began popping up around 1,200 years ago in the fertile river valleys of the American Southeast and Midwest. By far the largest of these was Cahokia, located a few miles from present-day St. Louis, Missouri, which at its peak hosted a population of up to 20,000 (similar to that of London’s at the time). Surrounded by a high wooden stockade, this inaugural U.S. city featured many plazas and at least 120 earthen mounds, the largest of which, known as Monks Mound, stood 100-feet tall and was built with some 14 million baskets of soil.

Meanwhile, just outside the wall, a ring of red cedar posts, dubbed “Woodhenge,” likely served as a sort of solar calendar. The city, a natural trade hub due to its position near the confluence of the Mississippi, Illinois and Missouri rivers, seemingly thrived in the 1000s and 1100s. But it allegedly started declining around A.D. 1200, right when a calamitous flood is known to have hit, and was long deserted by the time of Columbus’ arrival. In addition to the flood, researchers have fingered overexploitation of natural resources, political and social unrest, diseases and the so-called Little Ice Age as possible causes for Cahokia’s fall.

5. Easter Island

Ancient Civilizations that Mysteriously Collapsed: Easter Island
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Moai statues on Easter Island.

Setting out by canoe sometime between A.D. 300 and A.D. 1200, Polynesians somehow found and settled Easter Island, one of the world’s most remote places, located about 2,300 miles west of Chile. Even more remarkably, despite lacking wheels or pack animals—much less cranes—they managed to erect hundreds of giant stone statues, called moai, the largest of which stood 32-feet tall and weighed 82 tons. (Another moai, nicknamed “El Gigante,” stood 72-feet tall and weighed at least 145 tons, but never made it out of the quarry.) By the 1800s, however, every statue had been toppled, the population had crashed and the island’s chiefs and priests had been overthrown.

By analyzing charcoal fragments and the pollen in sediment cores, scientists have since discovered that Easter Islanders cut down almost every last tree, and that rats ate the trees’ seeds before the forest could re-germinate. This ecological catastrophe, which eliminated the ability to make rope or seagoing canoes and reduced the populace to burning grass for fuel, may have then ushered in a period of mass starvation and civil war. The arrival of Europeans only added to the decimation, starting in 1722 when the first Europeans to set foot on Easter immediately shot to death several islanders. By the 1870s, several waves of smallpox, along with a major Peruvian slave raid, had reduced the number of natives to roughly 100.

6. Greenland’s Vikings

Ancient Civilizations that Mysteriously Collapsed: Greenland's Vikings
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A statue of Leif Eriksson in the settlement of Qassiarsuk, the home of Erik the Red, in Greenland.

According to the Icelandic sagas, Erik the Red led a fleet of 25 boats to colonize Greenland around A.D. 985, not long after he was temporarily banished from Iceland for manslaughter. Setting up two colonies—a larger Eastern Settlement and a smaller Western Settlement—these Vikings herded goats, sheep and cattle, built stone churches that can still be seen today, and hunted caribou and seals. Thriving, or at least surviving, for hundreds of years, their population grew to roughly 5,000. Yet when a missionary expedition arrived in 1721, intent on converting them to Protestantism, it found nothing but ruins.

Archeologists have since determined that the Western Settlement failed around A.D. 1400 and that the Eastern Settlement was abandoned a few decades later. The onset of the Little Ice Age in the 14th century was almost certainly a contributing factor, as it clogged the route in and out of Greenland with sea ice and shortened growing seasons on what were already highly marginal lands. To make matters worse, the market collapsed for the Viking Greenlanders’ main export: walrus ivory. No one knows, however, what delivered the final deathblow.

Some experts believe they simply packed up and returned to Iceland or Scandinavia, whereas others think they starved to death, succumbed to the Black Plague or were exterminated by the Inuit, which had arrived in Greenland from Canada around A.D. 1200. At any rate, the Vikings were far from alone in their failure. At least three other societies have likewise perished on Greenland, including the Dorset, which for a brief time cohabited the island with both the Vikings and the Inuit.

READ MORE: DNA Suggests Viking Women Were Powerful Warriors