Great Wall of China - China

Rain clouds pass over a section of the Great Wall at Jinshanling, Hebei Province on June 10, 2012. A recent archaeological survey found its total length to be 21,196 km or 13,171 miles. (Ed Jones/AFP/Getty Images)

The sight of a building being torn down or renovated is a common one in any big city, and we've become all too accustomed to seeing homes and businesses torn apart by the elements, from hurricanes to tornadoes. Yet some of the most impressive structures in the world have remained standing for hundreds and even thousands of years. In the following pages, explore some of mankind's greatest architectural wonders that have withstood the test of time and can still be visited today.

The Great Wall of China is such an enormous structure it can be seen from space, and stretches across the country for such great distances that the Chinese government recently discovered the wall is thousands of miles longer than previously believed. Built and expanded on from the third century B.C. until the seventeenth century A.D., the Great Wall of China is an extensive defense project that crosses 13,170 miles of China's former borders. According to UNESCO, the wall shows theclash between agricultural and nomadic civilizations and demonstrates the excellence of ancient empires' military architecture. The wall isn't one unbroken structure, but multiple walls built during different empires, says Smithsonian Magazine.

Today most of the wall remains untouched, slowly succumbing to deterioration despite the best efforts of preservationists. But for tourists who want to hike along the ancient fortifications, there are several sections that have been restored and opened to the public, says CNN, including Badaling, Juyongguan, Mutianyu and Simatai (all near Beijing).

NEXT: The largest stone monument on earth

Great Pyramid of Giza - Egypt

The three large pyramids of Menkaure (L), Khafre (C) and Khufu loom over the horizon November 13, 2004 at Giza, just outside Cairo, Egypt. The three large pyramids at Giza, built by King Khufu over a 30 year period around 2,550 B.C., are among Egypt's biggest tourist attractions. (Sean Gallup/Getty Images)

Built 4,500 years ago, the Great Pyramid of Giza is a marvel of engineering that continues to perplex scholars and archaeologists. The pyramid, which was commissioned by King Khufu as a funerary monument around 2,550 B.C., took less than 30 years to build, despite requiring about 2 million blocks of limestone that each weighed around 2.5 tons, says PBS. It is still the largest and most accurately built stone monument on earth, rising at a constant angle of 52 degrees to a height of about 450 feet, reported Discovery. The largest pyramid is accompanied by two smaller ones and a statue of the Sphinx, a mythical creature that's part woman, part lion. According to UNESCO, the pyramid was a symbol of the sun and the sun god, Ra, and pharaohs were buried in these massive tombs in order to help them reach the afterlife.

There is currently a travel warning issued by the U.S. Department of State against traveling to Egypt because of continued social and political unrest, but if you journey to the country in the future, be sure to visit the Great Pyramids. The pyramids are located near Cairo, and a number of visitors can choose from a number of tours.

NEXT: An ancient structure that has undergone many transformations

Hagia Sophia - Turkey

A general view inside the Hagia Sofia (Aya Sofya) on February 23, 2012 in Istanbul, Turkey. (Dan Kitwood/Getty Images)

The Hagia Sophia, one of Turkey's most marvelous works of architecture, has undergone a number of transformations since it was first built 1,400 years ago. It began as a cathedral in the capital of the Byzantine Empire, Constantinople; became a mosque when Constantinople fell to the Ottoman Empire in 1453; and was finally converted into a museum by the Turkish government in 1943. The upheaval reflected in its history is also a part of its origin, says LiveScience, since the cathedral was built on the site of a church that had been burned down during riots. Despite how long it has survived, and the numerous cultures that have repurposed the Hagia Sophia and made it their own, the great cathedral-mosque is at risk from earthquakes and neglect, reported Smithsonian Magazine.

To see the Hagia Sophia while it's still standing as a cherished relic and symbol to various religious communities, head to Istanbul and check the museum's website for the hours the Hagia Sophia is open. Be aware that the museum is closed during the first days of Ramadan and Sacrifice Festivals.

NEXT: A temple dedicated to love and beauty

Taj Mahal - India

The Taj Mahal is seen on September 30, 2010 in Agra, India. Completed in 1643, the mausoleum was built by th Mughal emperor Shah Jahan in memory of his third wife, Mumtaz Mahal, who is buried there alongside Jahan. (Julian Finney/Getty Images)

Considered the "greatest architectural achievement in the whole range of Indo-Islamic architecture," says UNESCO, the Taj Mahal is a beautiful and popular destination for tourists visiting India. Built from 1632 A.D. till 1648 A.D., the white marble monument was commissioned by emperor Shah Jahan for his wife after her death, says PBS. The 145-foot-tall marble mausoleum is the best-known structure of the Taj Mahal, but the complex actually includes a red guesthouse, a mosque, and beautiful gardens, says LiveScience. For all that the Taj Mahal has remained the "Jewel of India," the environmental problems in Agra and other surrounding cities have been causing continuous damage to the complex's beauty -- for example, methane gas yellowing the marble -- reported Smithsonian Magazine. Others say the discoloration is normal, and may come from the thousands of visitors whose breath and hands touching the walls could cause discoloration.

While it may no longer be as pristine as it once was, the Taj Mahal is still a site worth seeing. To visit this monument to beauty and love, check out the visiting hours and fees on the Taj Mahal's website.

NEXT: A mystical collection of stones

Stonehenge - England

Visitors and tourists walk around the ancient monument at Stonehenge on March 2 2012 in Wiltshire, England. The English Heritage, managed prehistoric UNESCO World Heritage site, believed to have been constructed in 3000BC, has always been a 'must see' attraction for visitors to the UK. (Matt Cardy/Getty Images)

On the surface, a concentric collection of large stones may seem less impressive than the Great Pyramid or the Taj Mahal. But Stonehenge is considered one of the most impressive megalithic monuments in the world, and with good reason. The stones are up to 30 feet tall and weigh as much as 25 tons, yet somehow they were arranged in a specific formation 5,000 years ago, long before modern tools could have assisted the builders in erecting the menhirs. According to UNESCO, Stonehenge was built with unique engineering, including interlocking joints, and uses two different kinds of stones that might have been transported from great distances (as far as 240 km, or 150 miles, away). What makes Stonehenge an even more interesting monument is the enduring mystery concerning why it was built. Some scientists have speculated that the rocks are burial monuments, since burial goods have been found, and others say the site is a symbolic unification of Britain, since it would have required many workers, says LiveScience. More recently archaeologists were able to confirm that the monument was built along a landform from the ice age that aligned with the solstice axis, reported The Guardian.

Whatever the original reason for Stonehenge's construction might have been, casual tourists and reverent pagans continue to flock to the site in an effort to connect with the past. During the winter and summer solstices, hundreds of people in costume can be seen visiting the megaliths, as seen in the photos above.

NEXT: An arena the turned into a sea

Colosseum - Rome

The Colosseum has been standing for nearly 2,000 years, and while time -- and fires and earthquakes -- have taken a toll on the huge amphitheater, it remains a remarkable structure and a reminder of the great Roman Empire. Built in 80 A.D., the Colosseum was used to stage a variety of shows for the citizens of Rome. According to the BBC, around 50,000 spectators could crowd into the amphitheater at one time, and were seated based on social class, with the highest class sitting in the shade while everyone else sweated in the sun and sat far from the arena. The Colosseum was regularly used to show gladiators fighting against each other or against animals -- the animals were raised into the stadium from below using hand-operated lifts. But perhaps the most spectacular of shows were the naumachia, when the arena was filled with water and real warships that reenacted naval battles for the audience, says UNESCO.

Tourists wanting to visit the ruins of the Colosseum in Rome can find a guide or buy tickets to get inside, but they won't be able to drive by the site anymore. Al Jazeera recently reported that the roads going through Rome's ancient forum to the Colosseum are slowly being closed off in an effort to protect the monument from trash and pollution from car exhaust.

NEXT: An ancient British cathedral

Ely Cathedral - England

For over 1,000 years, Ely Cathedral has been a home to Christian worshippers in Cambrigeshire, England. Founded by Etheldreda, the daughter of the king of East Anglia, the cathedral was built around 673 A.D. and later sacked by the Danes, but restored in 970 to become "the richest abbey in England except for Glastonbury," says the cathedral's website. The large cathedral displays a number of architectural styles, but one of the most magnificent aspects of the building is the octagonal lantern, which replaced the Romanesque crossing tower in 1322, according to the BBC. Made of timber and supported by eight stone piers, the octagon was built by the king's own carpenter, William Hurley.

Visitors coming to the cathedral can still attend worship services throughout the year. The cathedral was particularly exceptional during June 2013, when it hosted the Ely Cathedral Flower Festival, complete with 75 displays and a life-size throne of flowers.

NEXT: The pinnacle of the Incan civilization

MachuPicchu - Peru

Tourists walk among the ruins of the Machu Picchu citadel, Cusco, Peru on July 6, 2011. (Cris Bouroncle/AFP/Getty Images)

Stretching across 80,000 acres in the Peruvian mountains, Machu Picchu is a magnificent creation of the Incan Empire. The city was built around 500 years ago when the Inca had no steel, iron or wheels, says National Geographic, yet examples of the Incas' mastery of engineering is apparent in the terraced farming plots, ramps, and walls that blend seamlessly into the mountainside. Scientists have uncovered a number of individual sections among the ruins, including a quarter for farmers, an industrial quarter, a royal quarter and a religious quarter, says UNESCO. Another unusual feature of Machu Picchu is a giant stone on a raised platform that towers over the plaza, which may have helped the Inca make astronomical observations, reported LiveScience.

For those interested in ruins and the history of the Incan Empire, Machu Picchu can be visited from Cusco, Peru. Tourists can choose from a number of packages, but should be prepared for the hike up to Machu Picchu and the high altitude of the site (nearly 8,000 feet).

NEXT: An improbably crooked tower

Leaning Tower of Pisa - Italy

A tourist takes a photo at sunrise on May 8, 2011, of the leaning tower of Pisa in an empty square. (Fabio Muzzi/AFP/Getty Images)

Built over the course of several hundred years the Leaning Tower of Pisa in Italy got its reputation for appearing less than stable almost immediately after being built. The reason for the bell tower's instability lies in geology. The terrain the tower was built on is a soft mixture of clay, shells and fine sand, meaning when the builders first started on its construction, they found the first third was already tilting 0.2 degrees, says the University of Houston. As the tower moved toward completion, it tilted even more, despite the builders' attempts to correct the tilt by adding different numbers of steps on each side, according to PBS. Over the centuries, it tilted up to 5.5 degrees, which should have sent the tower tumbling over, reported LiveScience, but restoration work restored the tower's stability at the turn of the 21st century. The tower is still leaning today, and still a popular attraction for tourists posing as if they were pushing the tower over, but it should be safe for the next two or three centuries, reported the Huffington Post.

NEXT: A Muslim fortress in North Africa

Salah Al-Din Citadel - Egypt

Located in the southern part of Cairo, this military defense complex was built in the late 1100s by Salah al-Din, one of the main opponents of the Christian Crusaders. The fortress houses three mosques, including the massive Mohammed Ali Mosque, as well as a military museum and a garden museum. According to the fort's website, the citadel also contains the Gawhara Palace, which was built in 1814 and is named for Mohammed Ali's last wife. The citadel protected Egypt's Muslim rulers for 700 years, says Lonely Planet, and is built on a limestone spur, providing a view of sprawling Cairo.

Visitors in Cairo can get to the citadel by taxi and visit every day for an admission of 50 EGP ($8 USD), says US News and World Report.

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