The next morning I caught a taxi north of Yulin, where the Great Wall ran through the desert. Tourists rarely came to see the wall here, because it was unrestored and the northern Shaanxi roads were so bad. There was no mention of the wall in my guidebook, but I had a Chinese map of the province that marked the ruins clearly.
The cabbie took me to a big Ming Dynasty fort that stood five miles outside of town, where Yulin's irrigated fields ended and the desert began. From the fort's highest tower the view stretched northward for miles. Occasionally, the barrenness was punctuated by a slice of green where water had found its way - a stand of trees, a lonely field - but mostly it was just sand and low brown hills and a vast thoughtless sky.
At nine in the morning the sun was already hot. I looked out at the empty landscape, at the hard low line of the horizon, and I realised why they had built the wall here. Even if there had been no Mongol threat, the terror of the land's monotony would be enough to make you build something.
The wall ran east and west from the fort. Westward it continued to its final stopping point at Jiayu Pass, in the mountains of northern Gansu province. Eastward the ruins ran to Zhonghai Pass, at the shore of the Yellow Sea. All told, the distance between these two endpoints was probably more than 1,500 miles, and Yulin was somewhere roughly in the middle; but the wall had never been fully surveyed and nobody knew the exact length. I stood there at the desert fort, looking out at the heat waves shimmering above the sandy hills, and I decided to go toward the ocean. I tightened my boots and walked east along the ruins.
Most of the wall was just a 3ft-high ridge of packed earth that had been worn down by the wind and sand. Every 200 yards or so I passed the ruins of a signal tower - a crumbling 20ft-high pile of dirt standing uselessly under the burning sun. I followed the…