Great Wall Of China

In 1644 the most formidable fortification on Earth was defeated by a man opening a gate. Wu Sangui commanded the Ming garrison at Shanhai Pass, where the wall runs down to the sea. A rebel army had just taken Beijing and the emperor had hanged himself. Wu, caught between the rebels to his south and the Manchu army to his north, chose the Manchus. He let them through. They marched to Beijing, founded the Qing dynasty, and ruled China for the next two and a half centuries. The wall that had absorbed generations of labour was, at the decisive moment, simply walked around by invitation.

Not One Wall

What gets called the Great Wall is really a family of walls built over more than two thousand years by different states, in different materials, along different lines. The earliest pieces date to the seventh century BC, when rival kingdoms fortified their borders against each other. After Qin Shi Huang unified China in 221 BC he had the northern stretches joined into a single frontier against the steppe nomads. Those Qin walls were rammed earth and rubble, and most of them have melted back into the landscape.

The wall in the photographs, the one with stone facing, crenellations and watchtowers striding over mountain ridges, is almost entirely Ming. Building ran from the late fourteenth century into the seventeenth, and accelerated after 1550, when the Mongol leader Altan Khan raided up to the suburbs of Beijing and embarrassed the court into a serious construction programme. The famous sections near the capital, Badaling and Mutianyu among them, belong to that era.

In 2012 China's State Administration of Cultural Heritage published the results of a multi-year survey that counted every surviving fragment from every dynasty: 21,196 kilometres. The Ming wall alone runs well over 8,000.

Rice In The Mortar

Ming masons bound their bricks with lime mortar cooked up with sticky rice soup. In 2010 a team at Zhejiang University confirmed the chemistry: amylopectin from the rice makes the mortar denser, stronger and more water-resistant than lime alone. Part of the reason the bricks have stayed put for six centuries is, literally, porridge.

Look closely at the bricks themselves and some are stamped. The marks record the year of manufacture, the kiln and the military unit responsible, so that a failed brick could be traced back to whoever made it. It was a quality-control system, and you can still read it while walking on the wall.

The wall was also a communications line. Beacon towers spaced along the ridges relayed warnings with smoke by day and fire by night, and a signal could travel hundreds of kilometres far faster than any rider.

What It Did Not Do

It never really kept anyone out. Mongols crossed it, Manchus were let through it, and after the Qing took power the wall stopped being a border at all; the dynasty had come from the other side. Maintenance ceased, and for three hundred years it was left alone. During the Mao era villagers were encouraged to take bricks for houses and pig pens, and in parts of the north you can still find barns built from Ming masonry.

The body count is not known. Estimates that a million people died building it are guesses, not records. The folk belief that workers were buried inside the wall has not been borne out by excavation; no human remains have turned up in the fill.

And it cannot be seen from space with the naked eye. China's first astronaut, Yang Liwei, said so plainly after his 2003 flight, and the national textbooks were corrected afterwards.

Now

The Great Wall has been a UNESCO World Heritage Site since 1987. The tourist sections near Beijing are heavily maintained, some to the point of being rebuilt. The rest is in trouble. A 2015 assessment by the China Great Wall Society estimated that roughly a third of the Ming wall had already vanished, and that much of what remained was in poor condition. In the west, earthen walls erode under wind and rain. In the east, visitors carry off bricks, farmers cut roads through, and in 2016 a stretch in Liaoning was "restored" by capping it in concrete, turning a crumbling ridge wall into a smooth cement path and prompting a national outcry.

You do not need to go to orbit to see the wall. For most of its kilometres, you may already be too late.