When we think of crumbled empires whose ultimate fates remain unknown to us, one of the first names to come to mind is that of the Mayan Empire. The Mayans once occupied the entire Yucatan Peninsula in what is today Mexico, Guatemala, and Belize. While the Mayan language is still spoken today throughout Mexico and Central America, the great civilization fell into decline before the arrival of European settlers to the region, and no one is certain why. Though climate change and internal conflict are among the most popular theories, over the years much more fanciful ones have also been concocted, including the one that forms the backbone of the odd 1959 science fiction film Caltiki–The Immortal Monster.

Though little is known about the people who once inhabited this large basin roughly 500 miles south of modern-day Cairo, or what may have become of them, we have discovered from archaeological sites in the area that the people here farmed, domesticated animals, and fashioned ceramic vessels more than 9,000 years ago. Among the most striking ruins that remain in Nabta Playa are stone circles resembling Stonehenge. These circles suggest that the people who once lived here also practiced astronomy. 

Considered by some to be one of the first urban settlements in the world, this ancient city in modern-day Turkey is in some ways most notable for its unusual architecture. Built without streets as we know them, the city was instead a sort of honeycomb of buildings, all connected together. The inhabitants used the rooftops to travel, and climbed down ladders to access their dwellings. Instead of having communal cemeteries, residents buried their dead beneath the floors of their homes. While it is theorized that the city's residents spread out into the rest of the region, its unique architecture and culture seems to have never been reproduced.

A scene of a scuba diver exploring a massive Egyptian statue may seem like something from the upcoming Tom Cruise Mummy movie, but it's actually archaeologists attempting to piece together the history of the city of Thonis. Once the gateway to Egypt, Thonis is now on the bottom of the Mediterranean Sea. Like the fabled city of Atlantis, Thonis sank beneath the waves. No one is quite certain why, though today archaeologists are attempting to excavate the remains of the sunken city. 

Not as old as Catalhoyuk, perhaps, but equally unusual is the underground city of Derinkuyu.  Derinkuyu is also located in what is now Turkey. Though it is not the only underground city in the region, it is one of the most extensive, reaching its greatest size sometime between the years 500 and 1000 CE. The underground city consisted of tunnels and rooms cut into the soft rock of the region, extending down for as much as 5 stories and housing 20,000 people as well as livestock at its peak. The subterranean city once offered respite from nearby enemies, even after it was used as a residence. It was fully abandoned in 1923, not to be reopened to the public until 1969.

Famous for its moai, the large stone statues that are virtually synonymous with its name, Easter Island is also known as Rapa Nui and was once home to a thriving and industrious, if not particularly large, population. By the time European explorers discovered the 63 square mile island, the population had declined dramatically, likely as a result of over-harvesting of the local palm trees. Whatever the actual reason, the origins and decline of Easter Island and its unique culture and statues have retained a lasting fascination ever since. 

Here in the United States, we often think of lost civilizations as being the purview of explorers of far distant lands, but many can be found closer to home. There are lost civilizations waiting just beneath the surface of even America's heartland. Just across the Mississippi river from St. Louis, Cahokia was once the largest known city in North America. Consisting of almost a hundred earthen mounds, many of which can still be visited today, as well as a huge central plaza, the city's inhabitants are known to have diverted the flow of the Mississippi river. No one knows why they abandoned the city around 1200 AD. Some researchers believe that two significant floods could have contributed to its decline and abandonment. 

The civilization that we call "Anasazi" left behind incredible pueblo cities cut into the cities of cliffs throughout the American Southwest, known now as the Foothills Mountain Complex. What they did not leave behind was a reason for their decline, or even their actual name. The name "Anasazi" comes from Navajo and means ancient enemies. Many contemporary descendants of this ancient civilization prefer the term Ancestral Puebloans. Whatever they were called, the Ancestral Puebloans once built great cities across the areas of Utah, Arizona, New Mexico, and Colorado, but by the end of the 13th century, the cities were largely abandoned. Popular theories about the decline of the Puebloans include drought, famine, war, and, of course, something to do with aliens. 

Today, Angkor Wat is one of the most famous and immediately recognizable ruins in the world. However, Angkor Wat was once simply part of a larger city, Angkor. The enormous temple complex—the largest religious monument on earth, occupying some 402 acres—was once a part of a metropolis larger than modern-day New York City. Angkor was the capital of the Khmer Empire. The city flourished during much of the Khmer rule, but fell into ruin 300 years before the end of the empire. While there are plenty of explanations for why the city may have slipped into ruin, what we still don't understand is the full extent of its original scope, or just how many people may have lived there at its height.

The myths surrounding Roanoke have become a significant part of American history and folklore. The so-called "lost colony" of Roanoke Island, located in what is now Dare County, North Carolina, has inspired countless storytellers since the colonists vanished without a trace. The only sign they were even there was the word "Croatoan" carved on a fence post. The significance of that single word—which was the name of a local tribe as well as a nearby island—has haunted tales of the Lost Colony of Roanoke as much as the strange disappearance of its more than one hundred inhabitants.

These are just a few of the once-great cities and civilizations that have disappeared seemingly overnight throughout the history of the world. For all that we seem to know the history of our planet, every day new mysteries are uncovered, and each answer seems, sometimes, to only beget more questions as we dig deeper beneath the surface of our globe.

Featured photo: Lukasz Maznica / Unsplash; Additional photos: Jim G / Flickr (CC), British Museum, Good Free Photos (Public domain), Robert Nyman / Flickr (CC), Ken Lund / Flickr (CC)