We know there are questions around travel amid the coronavirus (COVID-19) outbreak. Read our note here.

In a world filled with numerous architectural splendors, the Great Wall of China stands out for its sheer size. After all, it's the only man-made structure you can see from space, right? This is actually untrue (but we'll get to that later). Want to know more about this historic feat of engineering? Here are 10 things you probably don't know about one of Asia's most beautiful landmarks. Don't forget to put it on your list of must-see world wonders.

Construction Lasted Over 2,000 Years

Visitors walking at the Great Wall of China in autumn
Credit: Elisaveta Ivanova/ iStock

Parts of the Great Wall date as far back as the seventh century B.C, but it wasn't until the third century B.C. that a series of walls was combined to form a single defensive system. Construction on the wall continued over the following centuries and lasted until 1878, when the wall became less critical as a military defense fortification.

Repairs to various sections of the wall have been ongoing. In recent years, restoration work has been carried out to shore up decaying portions of the wall. According to news sources, renovation on the eastern part of the Jiankou Great Wall is expected to be complete by August 2020.

It's Over 13,000 Miles Long

The Great Wall of China at sunset
Credit: bjdlzx/ iStock

You already know it to be the world's longest wall, but did you know exactly how far the Great Wall of China extends? Although there have been various estimations about its length over the years, the Chinese government only released official numbers in 2012. The Great Wall is apparently about 13,170 miles long.

Sections of the Great Wall lie in more than 15 regions and provinces from its easternmost point in Hushan, Liaoning Province to its westernmost watchtower in Jiayuguan, Gansu Province. The most popular section of the wall (Jinshanling) is 80 miles northeast of Beijing, which is one city you won't want to miss on your vacation. The Great Wall's length, however, means you can get to Beijing using several routes. For example, you can access downtown Beijing through the Shuiguan pass, which is just east of Badaling.

It Was Mostly Built During the Ming Dynasty

The Great Wall China terracotta soldier
Credit: bjdlzx/ iStock

Although the wall was constructed over several dynasties, the bulk of what remains today was built (or rebuilt) during the Ming Dynasty from 1368 to 1644. The length of this portion of the wall is approximately 5,500 miles and includes the most visited sections of the wall today.

During the Ming Dynasty, China was under constant threat from both the Mongols and Manchurians. The wall became an invaluable part of the country's defenses and included over 1,000 forts and garrisons along its length.

The Mortar Was Made From Sticky Rice

Mortar on the Great Wall of China
Credit: Nikolaj Alexander/ iStock

Centuries after the Great Wall was built, scientists discovered that the mortar used by the ancient Chinese contained sticky rice. The rice strengthened the customary mortar mix made from slaked lime and made it more water-resistant.

As the mortar filled the gaps between stones and bricks, it acted to fortify the wall's entire framework. This innovation is a large part of why so many ancient Chinese structures are still standing today. It may also have been the first composite mortar (a mortar made of both organic and inorganic materials) ever created.

The Wall Is 33 Feet Thick at Its Widest Point

The Great Wall of China surrounded by lush green foliage
Credit: aphotostory/ iStock

The Great Wall varies in width and is actually 33 feet thick at its widest point and 2.3 feet thick at its narrowest. The widest part of the wall is in Juyongguan, while the narrowest part of the wall is in Zunhua City in Hebei Province. Generally, the Great Wall has a width of around 13 to 16 feet in most sections. This allowed 10 soldiers to march side-by-side when the wall served as a defensive fortification. That said, the Great Wall is wider at the base than the top. This upward contraction design is the reason for the wall's stability.

It Wasn't an Effective Defense Fortification

The Great Wall of China at sunset
Credit: hxdyl/ Shutterstock

Despite the wall's appearance as an impregnable defense that has withstood the test of time, it wasn't particularly effective at keeping out invaders. Throughout its history, the wall was breached by various Mongol and Manchu rulers including the infamous Genghis Khan in the 13th century.

During the Ming dynasty (which saw considerable resources allocated for the wall's renovation), Mongol raiders broke through the barriers to terrorize the locals. Though the Great Wall remains a symbol of Chinese power, it often proved ineffectual in keeping out invaders during the height of its use.

It Isn't a Continuous Structure

Closeup view of stairs at the Great Wall of China
Credit: Harry Portilla/ Shutterstock

Although our mental image of the Great Wall is probably one of a continuous structure winding its way across China, the reality is much different. The Great Wall is actually composed of various stretches of wall and watchtowers — often with gaps between. There are even areas where the wall is non-existent.

The original builders also made use of natural barriers to keep invaders out. As much as a quarter of the wall's length relied on features like rivers and mountainous ridges to keep the marauding hordes back.

You Can Run a Marathon on It

Woman walking along the Great Wall of China with a backpack on
Credit: Onfokus/ iStock

There are two major marathon events that take place along the Great Wall in the spring. The Great Wall Marathon is an international event that draws competitors from all over the world. It's usually held in May. Meanwhile, the Jinshanling Great Wall Marathon is held in April and offers a full marathon, half marathon, and 10K race (in case you're not up for the full 26.2 miles). This race also attracts runners from many countries. No matter which event you participate in, be prepared to navigate some of the most challenging — and rewarding — miles in the world.

Roughly 10 Million Visitors Visit the Wall Per Year

The Great Wall of China on a cloudy evening
Credit: Jeff_Hu/ iStock

As travel around the world increases, the Great Wall (like other popular destinations frequented by tourists) has seen its number of visitors grow. In fact, the most popular section of the wall has become so overcrowded that a daily quota was instituted in 2019 to manage the crowds. In previous years, the Badaling section of the wall saw as many as 100,000 visitors in a single day during its busiest times. Under the new quota, however, the number will be capped at 65,000 visitors per day.

It Can't Be Seen From Space Without a Magnification Aid

Aerial view of the Great Wall of China
Credit: loonger/ iStock

It turns out that the one thing you thought you knew about the Great Wall of China isn't true. Though it has long been heralded as the only man-made object you can see from the moon, the truth is that the wall blends into its surroundings too much to be visible from that distance. In addition, no astronaut has been able to catch a glimpse of the wall from the International Space Station.

Under certain atmospheric and lighting conditions, however, the wall can be seen in photographs taken from low orbit satellites. Even if the moon myth isn't true, the Great Wall is still an impressive sight — no matter where you catch a glimpse of it.