(CNN) — Ancient sites that have survived centuries, or even millennia, haven't fared so well in the 21st century.
The destruction at Palmyra in Syria, the Buddhas at Bamiyan in Afghanistan, and Nimrud's ruins outside Mosul in Iraq show how precarious the future can be.
Here are 23 places that offer glimpses into humanity's past, and the ingenuity that people brought to creating wonders without computers or heavy machinery.
While some are recognizable, others are easily accessible but lesser known.
Angkor Wat, Cambodia
So beautiful, so misunderstood. To get this sunrise shot, you're going to be fighting with a few hundred other tourists, all crowded around Angkor Wat's moat.
Alex Ogle/AFP/Getty Images
In the 12th century, the Khmer took their vast understanding of the known universe and sought to recreate it in miniature.
The result was Angkor Wat, a sprawling city designed to impress with meticulously arranged moats and towers, and walls covered in astonishingly detailed bas reliefs of Hindu deities.
The ancient Khmer took a broad view of scenes worth preserving.
While many are celestial or holy in theme, other murals detail mundane acts like preparing supper.
Angkor Wat stretches over 500 acres within the Angkor Archeological Park, a large area covering more than 150 square miles.
The main temple receives packs of tourists, but many lesser-known temples offer a chance to wander through old Khmer capitals, which were built from the 9th century onwards.
Visiting a city as beloved as Rome comes with particular hazards: the crowds, the cheesy souvenirs, the young men in full gladiator regalia waiting to pose for selfies with tourists.
And yet there are few places like it.
Take the Colosseum -- a nearly 2,000-year-old stadium in the middle of a modern city.
In the days of actual gladiators, 50,000 spectators would gather with the emperor for bloody contests to the death.
The Colosseum had the original retractable roof, a whizz-bang contraption called a velarium that used sail technology to rig canopies to shelter crowds from sun and rain.
And we've not even mentioned the Vatican, the catacombs or the Forum.
Istanbul loves to depict itself as the city straddling two continents.
What's most remarkable is the way the city straddles great periods of history that pile up and fold over themselves more naturally than anywhere else in the world.
Construction by successive empires from Byzantium to Constantinople to modern Turkey have bequeathed Istanbul an instantly recognizable skyline that merges elements from all those eras.
In the historic core around the iconic Hagia Sophia, the Byzantine-era Hippodrome circus sits a short stroll away from the Ottoman Empire's Topkapi Palace, which houses artifacts including Moses's staff.
Surrounding it all is a thriving modern city with top-notch dining, galleries and architecture that make Istanbul one of the world's most important cultural centers.
Less crowded than Angkor Wat and Machu Picchu, but just as magical.
Courtesy Nicolas Asfouri/AFP/Getty Images
Long hidden from international view by Myanmar's military government, the treasures of Bagan are returning to the spotlight as political reforms open up the country.
Here, over 2,000 Buddhist temples fill a plain along the Irrawaddy River, creating an ethereal landscape.
The crowds remain far smaller and more adventurous than the tour groups that fill Angkor Wat or Machu Picchu.
Myanmar is still finding itself after decades of civil war and international isolation, which makes this 11th-century temple city all the more magical.
The kingdom that reigned here was destroyed by earthquakes and Kublai Khan's invaders, but the quiet temples retain a spiritual air that's impossible to ignore.
Hidden in the jungles of Guatemala, Tikal was a Mayan citadel that reflects more than 1,000 years of cultural achievements beginning from 600 BC.
Jaguars and pumas prowl the surrounding wilderness, but the palaces, temples and plazas within the site represent some of the earliest pinnacles of human achievement.
The stepped pyramids are icons of Mayan culture that rise above the canopy.
Equally impressive are the sporting courts, temples and palaces that ring the main plaza.
Most of the ancient causeways that link Tikal's 3,000 structures have been cleared of vegetation, so visitors can now wander among the buildings much as the ancients did.
For first-time visitors, it's a shock just how close the pyramids of Giza and the Great Sphinx are to Cairo's chaotic streets.
With 22 million people, Cairo is one of the world's biggest cities, built around one of humanity's earliest urban centers.
Tombs at Giza date back 4,500 years, and the Egyptian Museum of Antiquities boasts an incredible collection from the Nile's earliest inhabitants.
But the slightly less ancient parts of Cairo are also rich with cultural treasures.
The current city was founded more than 1,000 years ago and has one of the world's oldest universities, a rich legacy of Islamic art, and Coptic treasures that are often overlooked.
Mohammad Reza Domiri Ganji's amazing photos of Iran are a great introduction to some of the country's rich cultural attractions. Among them is this literary institute in Shiraz, which has a reputation as a city of great poets and poems.
Mohammad Reza Domiri Ganji
As Iran slowly reopens to Western tourism, Persepolis is regaining its allure for visitors.
The city was founded in 518 BC by Darius I, ruler of ancient Persia's Achaemenian Empire, and grew in grandeur until Alexander the Great sacked it two centuries later.
Its most remarkable feature is an immense terrace of 125,000 square feet, partially carved out of Mount Kuh-e Rahmat (the Mountain of Mercy).
Rulers built ever more regal palaces, temples and halls around the terrace, complete with an underground sewage system and cisterns for fresh water.
Despite a series of protective walls, rising to 30 feet high, Alexander laid waste to Persepolis, whose ruins were only rediscovered in 1618.
Today, however, the city is one of the best examples of ancient architecture, especially for the slender columns that remain.
Kyoto cool: Japan's former capital preserves old traditions.
Buddhika Weerasinghe/Getty Images AsiaPac/Getty Images
If Tokyo represents the part of Japan obsessed with technology and the future, Kyoto is the part that rakes sand in Zen gardens and performs graceful tea ceremonies.
That's not entirely fair -- Nintendo is based in Kyoto, just one part of the city's thriving tech scene.
Perhaps closer to the truth is that as imperial Japan's capital for more than 1,000 years, Kyoto has found a way to respectfully preserve its old traditions while eagerly embracing the new as well.
More than 1,000 Buddhist temples and Shinto shrines temper the frenetic pace of modern life.
Since the city was largely spared bombing during World War II, most are still in use.
China has invested heavily in eye-popping modern architecture for its capital over the last two decades, but with a past that stretches more than 3,000 years, the city has a deep history providing a rich legacy of art, architecture and education.
Just visiting the city's six UNESCO World Heritage sites, including the Forbidden City and the Temple of Heaven, could take a week.
That would barely allow even a casual glance at the treasures inside the city's 144 museums and galleries, much less the alleyways of the hutongs, old neighborhoods reinvented and sometimes rebuilt as a trendy center of the Beijing's modern life.
Great Zimbabwe, Zimbabwe
From the 11th century, this kingdom in southeastern Zimbabwe grew into a trading empire that stretched through the African interior and across the Indian Ocean.
A royal complex grew on a hilltop, with drystone architecture creating a terraced palace with fruit trees, hidden chambers and stunning views over the king's dominion.
Below, the towering walls of the Great Enclosure surround a conical tower that's become a national symbol.
Overpopulation and deforestation caused the kingdom's collapse around 1450 -- historically bad timing as Europeans soon began arriving to find a weakened polity that was easier to subdue.
Great Zimbabwe remains a monumental reminder of Africa's achievements before colonialism.
Before the Sahara nearly swallowed the city and before French colonialists swept through, Timbuktu was one of the world's most important centers of learning.
The city's librarians guarded thousands of manuscripts, protecting them against the elements and violence.
Many of the manuscripts were evacuated to Mali's capital Bamako during the last period of separatist violence, but the city's unique mosques remain.
The three most important ones date as far back as the 14th century.
Their earthen architecture requires continuous maintenance, and caretakers today use the same techniques to preserve them as the original builders.
Machu Picchu, Peru
One of the most popular ancient cities in the world.
Nearly 8,000 feet high in the Andes, the Incan citadel of Machu Picchu retains an aura of mystery and majesty even as it welcomes thousands of tourists a day.
When the Spanish conquered Peru, Machu Picchu's very existence was kept secret from them.
The vast complex was revealed to the outside world in 1911, and quickly became the emblem of Incan achievement.
Built more than 500 years ago with drystone construction, the city sits on the saddle between two peaks rising above tropical jungles.
Clouds swirl among the ruins, whose significance remains largely unknown.
Terraces and ramps skirt the mountains, and the buildings appear to reflect the Inca's sophisticated grasp of astronomy -- though archeologists are still seeking to understand exactly how.
Athens traces its origins back 5,000 years, but the modern city has found remarkable ways to coexist with its iconic ancient monuments.
Below the Acropolis, walkable neighborhood streets wind past cafes and shops.
Many brim with tourist tchotchkes, but with enough art galleries and good eating mixed among them even the most jaded traveler will be satisfied.
Much of the city's social life centers around the monuments, turning the stunning ruins of classical Greece into casual backdrops of Athenians' every day lives.
This region between the Dead Sea and the Red Sea has been inhabited since the earliest days of humanity, but around 300 BC the rulers of the Nabatean Kingdom laid their claim to cultural greatness by carving magnificent buildings into red sandstone cliffs.
Gorges and canyons surround the ancient city, creating a maze of passages that helped keep it secret from Europeans for centuries.
Adding to the mystique: no cars are allowed in the ancient city.
Visitors who don't want to walk have to hire a camel, donkey or carriage to get around.
Like Petra, Ellora was carved out of the mountains in the area of Maharashtra.
But this Indian monastic complex remains relatively unknown.
The temples include Hindu, Buddhist and Jaina holy sites that were built over 400 years, beginning in the 6th century.
The most impressive has the uninspiring official name of Cave 16, known informally as Kailasa, and the entire temple was carved out of a massive rock face.
The double-story entrance leads to a courtyard surrounded by a three-story arcade lined with columns. The building is twice as large as the Parthenon.
Inside, images of Hindu deities were sculpted from the stone, with two life-size statues of elephants in the courtyard.
In addition to the famed Terracotta Warriors, Xian's well-preserved city wall is worth a visit.
In the Silk Road's heyday, Xi'an was where the journey west began.
For more than a millennium, Xi'an was the capital of ancient Chinese dynasties.
Now it's most famous as the home of the Terracotta Warriors -- thousands of clay soldiers that fill the mausoleum of Emperor Qin Shi Huang. Each is standing, ready for battle, but they had been buried underground for 2,000 years.
Workers digging a well stumbled upon one of the soldiers in 1974, leading to one of the great archeological finds of all time.
Excavation is still underway, and more recent work has discovered statues of dancers and acrobats, giving a new sense of the grandeur of Qin's ambitions.
In ancient days, the Tamil city of Madurai was an important capital that traded with Rome and ruled over its part of India until the British conquest.
At its heart is the Meenakshi Amman Temple, one of India's holiest sites that draws one million visitors for its annual festival in April and May.
Although the city is 2,500 years old, the original temple was sacked during a period of medieval invasions.
The current structure was built in the 1600s, with 14 gateway towers guarding the shrines covered in vibrant colors.
The Thousand Pillar Hall is carved with the mythological yali, a creature with the head of elephant on the body of a lion.
Poverty Point, Louisiana
Largely unknown even to Americans who live nearby, Poverty Point in Louisiana achieved the height of early human civilization in North America.
It's named for a plantation located nearby, as the original name has been lost to time.
The city probably reached its greatest heights around 3,500 BC, when its people carved out six concentric, elliptical ridges from the earth.
They also built five large mounds, erected causeways and established docks that gave them easy access to the Mississippi River.
That helped turn this site into a trade hub for a network that stretched over hundreds of miles.
Though the area was developed by people who hunted, fished and gathered wild plants for food, farming hadn't been established yet.
Borobudur is the most visited tourist attraction in Indonesia.
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Foreigners tend to think Indonesian travel as centering on the tranquil island of Bali or the hectic capital Jakarta.
But the most-visited site in this mainly Muslim nation is the ninth-century Buddhist temple complex of Borobudur.
The temple was completed in the early 800s, but much of its history has been lost.
For centuries, it lay covered under volcanic ash, and then the surrounding jungle grew over it.
In the early 1800s, locals told a British governor about its existence.
In the 1970s, UNESCO guided a restoration that brought Borobudur to its current glory.
Now the stupas once again rise among the green valley. Because the complex was buried for so long, most of the artwork remains intact.
This Buddhist monastery was built in the first century, but fell out of use a mere 600 years later.
Takht-i-Bahi's location on a hilltop protected it against a succession of invasions, until the British came through and hauled off most of the surviving treasures to keep in the British Museum.
What remains are three stupas and the court surrounding them, as well as meditation chambers once used by monks who brought Tantric traditions to this part of the world.
In the popular imagination, Tantric practices revolve around sex.
Takht-i-Bahi is a reminder that Tantra is so much more.
Archeologists believe this prehistoric stone circle could have been built as much as 5,000 years ago.
There's general agreement that the area was a burial ground, but no one really understands how the giant stones were brought in from as far away as Wales.
Construction may have taken place over 1,500 years, at a time before the wheel had arrived in England.
The capital of the old Ethiopian Empire, Gondar suffered under repeated invasions from the mid-1800s.
However several castles and churches remain, offering a look into one of the few medieval African cities that's still alive today.
Tourism here is not heavily developed, so wandering among these royal walls feels more like time travel.
The city is often called Africa's Camelot, which seems unfair.
Gondar is the center of a culture that's connected to the broader Christian world and completely distinct from it.
The capital of the world's first empire, Nimrud ruled over Assyria from around 880 BC, though humans settled the area thousands of years before.
Excavations since the 1840s revealed a formidable city with massive palaces and temples to gods of war and writing.
Ivory furniture, carved stone slabs, gold jewelry and crowns were all buried inside.
The art and architecture are irreplaceable, but many have been destroyed in just the last few months as Nimrud sits just outside Mosul.
After the Islamic State occupied the area, they brought in bulldozers to raze the ancient monuments. No one is exactly sure how much is left.