Narrator: Once powerful and populous centers of influence, these cities are now classed as ruins.

All lost, one way or another, to the ravages of time.

But in their rediscovery, these lost cities are providing clues not just to how our ancient ancestors existed in the world, but how they made sense of it.

These are cities like Machu Picchu, Pompeii, Petra, and Angkor.

Full of magic and mystery, the quest to discover and explore these forgotten sites reveals tantalizing glimpses of the beliefs, aspirations, fears, and hopes of humanity throughout the ages.

And, in the fall of these once-great civilizations can be found cautionary tales for our own.

Though collapsed in parts, these sites remain as towering examples of the world's greatest lost cities.

Cusco.

An ancient city perched amid the Peruvian Andes.

Today a vibrant tourist hub, 500 years ago, this was the high altitude capital of an empire, the likes of which the world had never known.

This was the home of the Inca.

Between the 13th and 16th centuries, the Inca ruled over an area twice the size of modern japan.

They were masters of technology, diplomacy, and military strategy.

A powerful and advanced culture unrivaled in the americas for hundreds of years.

But all that changed with the coming of strangers in the early 16th century.

Spanish conquistadors had arrived carrying superior steel weaponry and foreign diseases, the combination of which quickly and ruthlessly overwhelmed the indigenous populace.

As the Spanish invaders occupied Cusco, the once-mighty Incan empire was forced to retreat to remote strongholds high in the surrounding mountains... Where they were lost to time.

Less than 100 kilometers northwest of Cusco, across the densely forested Peruvian Andes, lies the mysterious ruins of Machu Picchu.

An ancient Incan citadel which, until recently, had fallen into myth and legend.

Perched high above the winding Urubamba river, Machu Picchu lay lost to the world and in increasing disrepair for over 300 years before its rediscovery in the early 20th century.

In 1911, American academic and explorer Hiram Bingham was led to the lost city by local farmers that had been unassumingly tending the site's steep terraced slopes for generations.

Amazed by the scale and grandeur of this revelation, the pioneering bingham reintroduced Machu Picchu to the world.

Today, thanks to bingham's highly publicized expeditions, Machu Picchu is one of the most famous archeological sites on the planet.

Breathtaking not only for its stunning views, but for the fact that it's situated nearly 2,500 meters above sea level.

Built around 1450, at the height of the Incan empire, Machu Picchu was designed to impress.

While there is still much debate as to the original purpose of this citadel, today it's widely believed to have been a sacred site.

Both its elevated positioning and architectural style are thought to pay homage to the sacred Andean peaks, known as Apus, that dominate the natural world around it.

Within the complex, pride of place was given not to residences but to temples, precisely placed in accordance with Incan cosmology.

The temple of the sun integrates manmade structures with the peak's natural rock formations.

It's situated on one of the site's highest points and was not only used for sacred rites and rituals but also as a vantage point for the Incas' fascination with astronomical studies.

This auspicious site was dedicated to the worship of the highest Incan deities and reputedly the exclusive purview of priests and high nobles.

Several windows within the temple's tower align with the summer and winter solstices, fundamental elements of the Incan calendar.

The enigmatic ritual stone at the heart of Machu Picchu, the Intihuatana, was thought to grant a special energy to those who approached it.

The Inca believed that this sacred stone anchored the sun to the Earth during its annual path across the sky, and was used to precisely pinpoint the date of the northward and southward equinoxes.

This construct was more than a religious icon.

It was an early scientific instrument.

The Intihuatana speaks to the ingenuity of an advanced society that reached well beyond its time.

Although fascinated with the heavens, the Inca were also a very grounded people, the evidence of which continues to stand triumphantly in the structures they built over 500 years ago.

In architectural terms, Machu Picchu is a timeless example of exceptional craftsmanship.

Seismic activity in the region rendered traditional mortar useless.

The great minds of this industrious culture were forced to think outside the box.

To create the dry-stone structures that define the complex, the Incas first mined local granite blocks before masterfully shaping them to fit together seamlessly in a manner that could withstand the geological rumblings of the region.

Built without the aid of iron tools or beasts of burden, this masonry technique has proved a feat of ingenious engineering.

These remarkable stone walls have transcended the bonds of time and continue to stand as an enduring legacy of this ancient but advanced culture.

In Machu Picchu, Hiram Bingham thought he had discovered the last refuge of the Incan empire.

But today, most scholars believe that another site, discovered by bingham in the same year, in the nearby valley of Vilcabamba, to be the scene of the Incas' final stand against Spanish rule in 1572.

Machu Picchu was once the sacred heart of a great empire, before it fell to another.

Reclaimed by the steamy highland jungles, it's withstood the ravages of time and today stands as a monument to a culture well ahead of its time.

Mapping the skies and mastering the Earth, the legacy of the Incan empire lives on in the lost city of Machu Picchu.

Nestled between jordan's great rift valley and the Dead Sea... Lies a rare treasure hidden in plain sight.

The lost city of Petra.

It's thought that the roots of this ancient arabic city date back to the fourth century bc.

Once the bastion of a powerful trading empire, Petra remained lost to European knowledge for centuries, until its rediscovery by Swiss explorer Johann Ludwig Burckhardt in 1812.

Today, it's hard to believe that such a conspicuous manmade wonder could have remained concealed from the modern world for so long.

It wasn't just how this secretive complex was built, it was where.

Petra resides along the fringes of Jordan's Ard as Sawwan Desert and is solely entered through what's known as the Siq, a narrow gorge formed by a winding split in the weathered sandstone.

The rock-cut architecture blends in seamlessly with the natural surrounds, astonishingly carved from the solid stone of towering cliff faces.

What's perhaps most surprising about these powerful and enduring structures is that they were built not by a great empire, but a nomadic people.

Ever on the move, the Nabataeans were arabic shepherds and traders.

But in the late fourth century BC, as trade routes began to solidify between the red sea and the persian gulf, the nomadic tribe set about dropping roots... Establishing a regional hub to capitalize on the growing stream of passing merchants.

The Nabataeans rapidly acquired wealth and soon focused their attention on the construction of Petra.

The tribe was skilled in the art of stone carving, and bent the harsh face of the desert to its will, erecting marketplaces, temples, theaters, and tombs.

And by strategically harvesting rainwater, the Nabataeans were able to cultivate crops in the most barren of soils.

In essence, they had created a desert oasis.

As word of this strategically positioned city and its wealth began to spread, the Nabataeans relied upon the siq to guard their hard-won spoils.

The narrow entrance first served to conceal Petra's discreet location from unwanted attention.

Failing that, it was used to inflict heavy causalities upon invaders caught in its exposed bottleneck.

For more welcome visitors, the reception was decidedly less daunting.

For nearly 500 years, Petra was the intersection of major trade routes linking the east with the west.

Upon entering the naturally fortified site, merchants visiting the hub of this great trading empire would have been awestruck by the rose-red city, carved into the crystalline mountains that hid and protected it.

The wide open spaces that now lie at the feet of these towering cliffs were once cluttered with the mud brick houses, inns, and stables of a vibrant and prosperous city.

But today, only the exquisitely hewn rock facades speak to its once-influential status.

By the early years of the second century, Petra had become a victim of its own success.

The prosperous trading hub was absorbed by the expanding roman empire as part of its arabian campaign in the year 106 AD.

Although Petra initially flourished under Roman rule, it was weakened by the establishment of sea trading routes, and soon fell into rapid decline.

In the years that followed, Petra endured as a religious center until a series of devastating earthquakes between the fourth and fifth centuries laid waste to most of its buildings and the city's vital irrigation systems.

No longer the desert oasis it once was, the remaining residents abandoned the once-powerful trading enclave to the sands of the desert, which held on to its tightly guarded secret, concealing this manmade wonder from the wider world for centuries.

Myanmar.

Until recently, this Southeast Asian country was a lost nation.

A global pariah, largely closed off to the rest of the world.

But with the slow liberalization of its military government, the country has once again opened its doors, allowing visitors to rediscover this golden land.

At its heart lies the ancient temple city of bagan.

Straddling the banks of the Irrawaddy River, Bagan stretches 25 square kilometers across the arid plains of central Myanmar.

Once the capital of the kingdom of bagan, the realm that first united the regions of modern-day Myanmar, this was the center of an empire that for 200 years was among the most powerful in Southeast Asia.

Founded in the ninth century ad amidst a number of competing city-states, bagan quickly grew in wealth and power, and by the late 10th century, had become the predominant authority in the region.

At its height, between the 11th and 13th centuries, more than 10,000 temples, shrines, pagodas, and monasteries were constructed within the kingdom's capital, all of them dedicated to Buddhism.

And, for a religion that abhors violence, it's no small irony that many of these sacred sites were built by slaves captured as the spoils of war with neighboring kingdoms.

As bagan grew in stature and notoriety, it was transformed into a cosmopolitan hub of culture and learning.

Centers were erected for religious studies, astronomy, alchemy, medicine, and philosophy.

Driven by an insatiable desire for growth, the city's elite paid little heed to the concept of sustainability.

By prioritizing timber groves over food crops, the population slowly began to starve, leaving the capital in a weakened state.

By the mid-13th century, the kingdom of Bagan had caught the attention of the Mongol empire to the north, a moment in history that would seal Bagan's fate.

Subjected to repeated mongol invasions, the king and his court reluctantly fled by boat down the Irrawaddy, leaving the glistening city in their wake.

Although the Mongol hordes left the city largely unscathed, the once-proud capital never regained its former glory.

While much of the citadel fell into ruin, some of the grandest structures were maintained by locals, surviving the years as sites of holy pilgrimage.

Today, Bagan continues to draw a crowd, and the best way to appreciate the magnitude of this once-lost city is from the air.

After centuries of neglect, wars, floods, and earthquakes, over 2,000 temples still stand here, echoing an ideology that continues to remain central to Myanmar's identity in the modern era.

Situated around 300 kilometers from Guatemala's modern-day capital Guatemala City, Tikal National Park is a world heritage listed site.

Spanning over 575 square kilometers, the park encompasses wetlands, savanna, and tropical forest.

And hidden deep within its rugged wilderness lies the ancient Mayan city Tikal.

Mesmerizingly beautiful, the surrounding jungle is also inhospitable and unforgiving.

The area is home to poisonous snakes, insects, and fearsome jungle cats and is noted for poor soil quality and high annual rainfall.

To survive here is to conquer the elements, to fight an unending battle with an unrelenting enemy.

And that's precisely what the Mayans did when they built this place.

Tikal was the largest and oldest of the Mayan city-states, and for centuries was one of the most influential political and religious centers of Mesoamerica.

The Mayans are considered one of the greatest ancient civilizations, and certainly amongst the longest lasting.

They occupied a wide territory across much of Central America and southern Mexico.

The Maya, notorious for the practice of human sacrifice, are also famed for their advances in mathematics, astronomy, and, of course, architecture.

Tikal's metropolitan center covered an area more than 140 times larger than today's Vatican City.

While the citadel itself supported a population of around 50,000, the surrounding villages and farms played host to tens of thousands more.

It was a big city, with big-city problems, like how to feed its people.

Although the region lacked natural resources, Tikal managed to generate wealth by utilizing its extensive network of waterways as trade routes to and from the coast.

They facilitated trade in food, textiles, tools, and precious metals amongst other central and south American peoples, and in doing so, profited greatly.

As Tikal grew over the centuries, those that added to the city chose not to demolish old structures but to build over and around them.

As a result, beneath the surface lies a wealth of historic constructs, each smaller and older than the last.

Since Tikal's rediscovery in 1848, only an estimated 15% of the ancient city has been excavated.

But even within this small area, over 200 monuments have been revealed, creating opportunities for archeologists to decipher their long-forgotten histories.

The jungle, which has concealed the site for nearly a millennia, continues to hold on to many of its mysteries, among them the story behind the city's famed twin pyramids.

These pairs of flat-topped pyramids are thought to have been built to mark specific periods within the famous mayan calendar.

To date, 9 such complexes have been identified here, more than any other Mayan site, but their exact purpose remains unclear.

The reason behind Tikal's seemingly sudden abandonment around 1,100 years ago has also been lost to the annals of time.

Drought, deforestation, war, and social upheaval have all been proposed as possible factors in the great city's demise.

What is certain is that the ultimate victory was claimed by Tikal's oldest foe, the jungle... Which first invaded and then concealed the city from the world for centuries, relegating it to the realms of myth and legend.

Even now, the old adversary waits patiently, ready to reassert itself if only given the chance.

Throughout history, many of the ancient world's greatest cities have fallen into rack and ruin.

Slowly obscured by the relentless creep of jungle vines, or the steady invasion of desert sands.

But not all were consumed by the ravages of time.

The tomb of one fateful port town was sealed overnight.

240 kilometers southeast of Rome is the excavated port city of Pompeii.

Pompeii rests forever in the shadow of mount Vesuvius.

The only active volcano on mainland Europe, Vesuvius is still considered to be amongst the world's most dangerous.

It's famed for a cataclysmic eruption in 79 AD that condemned Pompeii's ill-fated future to that of tomb and time capsule.

[rumbling] [indistinct shouting] this site has been under excavation for almost 3 centuries.

Since archeologists began reclaiming the city in 1748, Pompeii has provided an extraordinarily detailed and haunting picture of a long-lost era.

Reporter: The ill-fated city of Pompeii, destroyed by an eruption of mount Vesuvius in 79 AD, continues to emerge from the shroud of lava that has been its pall for centuries.

Careful digging has uncovered remarkably preserved landmarks.

Narrator: The electoral notices and graffiti on the walls to the loaves of bread and jars of fruit on the shelves of people's homes, everyday objects that have been preserved through the millennia thanks to a lack of air and moisture within their rocky crypt.

Prior to its burial nearly 2,000 years ago, Pompeii was booming... Prospering as both port city and resort town.

As thousands of Roman holidaymakers flocked here during the summer months, ancient Pompeii was, in a sense, not too far removed from the tourist hotspot we see today.

The city was humble but affluent... Its wealth evidenced in its public buildings, which included an amphitheater so magnificent that it reputedly inspired the iconic design of the Colosseum in Rome.

And what ancient port city would be complete without its brothels, servicing mariners and tourists alike?

But when lewd frescoes, such as these, were accidentally uncovered in 1599, their conservative discoverers promptly reburied them behind ancient walls, where they remained for several more centuries before their eventual rediscovery in the late 1700s.

The most profound, and confronting, artifacts to be unearthed here are the haunting remnants of Pompeii's doomed citizens.

Archeological evidence suggests they were overcome by heat and suffocated by toxic gases before they could escape the volcanic ash that would entomb them.

As if frozen in time, today, plaster statues made from the molten cavities left by their bodies recreate stark images of their last moments in this world.

It remains a mystery as to why around 2,000 of Pompeii's citizens elected to remain within harm's way, while the vast majority fled to safety.

What is known is that the extent of the damage and the memory of the tragedy was so great that no attempts were made to reoccupy the area.

In many ways, Pompeii was an unremarkable city.

But for its tragic fate, it may have escaped the spotlight of history forever.

Ironically, the catastrophe which destroyed this town has preserved its memory for the ages, unlocking a window to a reality long gone, but not forgotten.

Established in 1325 upon the site of an ancient Aztec prophecy, today, Mexico City stands as the oldest capital in the Americas.

With a population of over 10 million, this buzzing metropolis is also one of the world's largest.

But just 50 kilometers northeast of here lie the ruins of a far older capital, one with mysterious roots and equally grand claims to fame.

Hidden within Mexico's valley of the damned, this is the ancient city of Teotihuacan.

Despite being the nation's preeminent archeological site, there is much that's still unknown about its history.

It's thought that when the ancient Aztec discovered the ruins of this expansive citadel, the site had already been abandoned for centuries.

Awestruck by their find, they named it Teotihuacan, meaning "great city where men became gods."

the Aztec claimed a common ancestry with the city's founders and, as they revitalized the site, adopted elements of their culture.

Clues to the city's original name and the identity of the mysterious ancient people who built it are yet to be deciphered from the site's surviving hieroglyphs.

Teotihuacan spans more than 30 square kilometers, and, at its peak in the mid-fifth century, was home to over 150,000 people, making it not only one of the largest settlements in the known world, but also amongst the most powerful.

Recorded in ancient mayan writings throughout Mesoamerica, the influence of Teotihuacan is said to have extended well beyond its borders.

But the question of how they achieved this status remains.

The notable absence of fortifications and military structures within the city has been the subject of much debate.

How could a people wield such power without the use of military force?

Perhaps the answer lies in the Aztecs' savvy use of economics.

Teotihuacan controlled trade routes from present-day Arizona to El Salvador and both the oceans that flank Central America.

By the mid-fifth century, it had become a grand center of culture and commerce.

In today's terms, it was the New York of the ancient Americas.

The city's broad central thoroughfare, ominously titled the avenue of the dead, is dominated by the impressive pyramid of the sun.

Weighing more than 10 times the Empire State Building and comparable in scale to the largest pyramids of Egypt, this behemoth structure was built in accordance with astronomical beliefs held sacred by its ancient architects.

The walls of the pyramid were once plastered with locally sourced lime and adorned with brightly colored murals of jaguars, snakes, and celestial constellations, but most have long since eroded.

The ruinous temple on top of the pyramid has held onto the secrets of its original purpose despite extensive study.

Archeologists have found more success among the other major civic structures lining the avenue of the dead.

They've uncovered evidence of systematic burning within palaces and temples, a find that's fueled speculation that Teotihuacan's abrupt collapse around 750 AD could have been brought about by a popular uprising against the ruling classes.

What's perhaps even more intriguing than the mystery of Teotihuacan's demise is the question of how this lost city, hand built without beasts of burden or the wheel, rose from nothing, more than once, to dominate Mesoamerica so successfully and for so long.

Built on the banks of the Mekong River, Phnom Penh, the modern-day capital of Cambodia, rose to prominence during the 15th century, when the reigning Khmer King moved his court here from an ancient site much further upstream.

Hidden amidst the steamy jungles of Southeast Asia, these ruins, known as Angkor, once formed the heart of the rich and powerful Khmer empire.

About 300 kilometers north of Phnom Penh, until the early 20th century, the nearby township of Siem Reap was nothing more than a sleepy jungle village.

Today, a hive of activity, it owes its reinvention to a remarkable discovery made by French naturalist Henri Mouhot in 1860.

While in search of rare birds and insects within dense jungle, mouhot stumbled across a number of crumbling ruins shrouded in creeping roots and vines.

This humble discovery would ultimately lead to the uncovering of an ancient citadel, which at its peak covered an area larger than modern-day berlin.

In the ninth century, Angkor formed the epicenter of a vast empire.

Prior to the industrial revolution, it was the largest city on Earth.

Which is no small feat, given the region in which it was built was defined by the extremes of drought and flood.

But the ancient architects of this citadel transcended these environmental limitations with an ingenious irrigation system that provided reliable water for drinking and agriculture.

At its zenith, the Khmer ruled over much of Southeast Asia.

And as the jewel in the empire's crown, Angkor became a symbol of wealth and power.

The city exalted local culture, religion, and art within magnificent architectural structures, the likes of which the world had never seen.

But, as history dictates, the mighty, too, will fall.

By the 15th century, the Khmer empire, suffering a steady decline in social and political power, was beginning to wane.

In 1431, sensing Angkor's weakening state, a neighboring siamese empire sacked the city, forcing Angkor's population of roughly one million to abandon the citadel to the whims of the jungle.

By the time henri mouhot brought the lost city to the attention of the west, the tight grip of the jungle was all that was keeping many of the remaining structures together.

But one complex within the sprawling ruins, although neglected, was never truly abandoned.

Taking pride of place above all others, this is the Khmer's crowning architectural achievement-- Angkor Wat.

Established in the early 12th century, this expansive temple was dedicated to the Hindu god Vishnu.

Its remarkable Khmer temple architecture was designed to represent Mount Meru, home to the deities of Hindu culture.

Although one local legend states that a divine architect erected this sacred site in a single night, historical evidence suggests it was commissioned by King Suryavarman ii as his state temple and eventual mausoleum.

But not long after his death in the mid-12th century, the kingdom's population gradually converted to Buddhism... And Angkor wat was transformed into a center of Buddhist worship, as it remains today.

This iconic temple has not only survived the tests of time, but thrived within them.

Today, it forms the centerpiece of Cambodia's national flag, a salient symbol of the nation's proud history.

To this day, Angkor wat remains the largest religious monument ever built.

The years have taken little away from its wonder and it continues to demand attention.

Nearby, Angkor thom stands surrounded by an imposing moat.

Once stocked with crocodiles to deter invaders, the moat has more recently served to hold back the invasion of the jungle.

With gateways built wide enough to accommodate the passage of regal elephants, Angkor thom was the royal, religious, and administrative center of the capital.

Its defensive walls contain an area larger than that of ancient Rome.

Within this walled complex stands the otherworldly Temple of Banyon.

The last state temple to be built in Angkor, and the only temple primarily dedicated to the Buddhist faith, Banyon Temple was commissioned in the early 13th century by King Jayavarman vii.

The temple's impressive carved faces, sculpted from large granite stones, are representations of the Bodhisattva, a Buddhist symbol of compassion... And are thought to have been modeled in the king's own likeness.

The elaborate carvings adorning the walls are some of the finest examples of the period, immortalizing the lives of the Khmer kings, and people, who have long since departed.

The scale and splendor of Angkor owes much to a succession of Khmer kings attempting to outshine their predecessor... Each vying to build bigger and grander religious monuments.

Monuments that upon their death would become their mausoleum.

It was this blind ambition that eventually sealed Angkor's fate.

The city grew bigger and more populated than the surrounding environment could support.

In the 15th century, prolonged periods of drought, foreign invasion, and political unrest brought about the eventual demise of this once-powerful kingdom.

Mysterious, awe-inspiring, and illuminating, separated across the ages, in their rediscovery these great lost cities remind us that ancient civilizations were not so far removed from our own.

Built in accordance with the beliefs of their day, all had distinctive social structures, characteristic customs, art, and architecture, and all fell to forces greater than themselves.

Yet it was time, the great leveler, that ultimately relegated all to obscurity.

And perhaps the greatest lesson we can take from the world's greatest lost cities is to make the most of the time we have while we have it.