At Dunhuang, 70 km from the Gansu-Xinjiang border, I was preoccupied with resuming my quest for what I called the "wooden Wall", built primarily of branches of Chinese tamarisk, a tough desert shrub.

Built in 110 BC during the Han Dynasty, this unique section of the Great Wall was first photographed in 1907 by Aurel Stein, the British archaeologist who gained notoriety for making off with cartloads of ancient manuscripts from Dunhuang's "Caves of 10,000 Buddhas". Yet few people know about his survey of the Han Wall.

William Lindesay at the Han Dynasty "wooden Wall" with the 1907 photograph. Piao Tiejun

According to Stein's book, Serindia, the wooden Wall was "remarkably well preserved, 233 m in length and more than 2 m in height".

Even though mine was a scan of Stein's original photograph, it was looking soiled enough. I'd used it the previous year, when for two days I searched in vain for this elusive desert location.

This time I was accompanied by Piao Tiejun, a volunteer photographer for International Friends of the Great Wall, and armed with GPS data for the probable location of the section. With spadework, Jeep access may have been possible, but I did not want to spend the day road building, nor damage the fragile desert ecology, so I chose to hike in.

We set off at dawn. Mosquitoes scrambled as we emerged from the Jeep amidst the clumps of tamarisk that flourish beside the hazardous salt marshes.

The "go to" page of the GPS indicated we had round trip of 29.5 km ahead of us. Through the binoculars I spotted a solitary tree, directly on our bearing. "I bet that's 4-5 km away," I said to Piao.

In fact, the "tree in the middle of nowhere" was 25 minutes' march away. It was a Huyang, or Euphrates Poplar. A proverb says it grows for 1,000 years, dies, but remains upright for another 1,000, and then falls. Its wood is said to be useful for a third 1,000. We huddled in the shadow of its small canopy to eat dried apricots.

Crunching across the encrusted surface, the kilometers counted down at a rate of between four and five per hour. There were signs of life, except for a distant glimpse of antelope bolting into the enveloping mirage. Then, as the GPS registered 400 m to go, I spotted a long, flat-topped object amidst sand dunes. Was it the wooden Wall?

It was indeed, but my elation was only brief. The Wall seems to have suffered over the last century. It was lower, banked with windblown sand and fragmented. We'd also arrived an hour too late. The Wall was backlit, ruling out any possibility of getting a good quality comparative image.

Suddenly I realized this Wall had two faces. I found a crossing point and, as if guided by destiny, went right to the spot where Stein had stood, in April 1907, to take his photograph.

Incredibly, this 2,100 year-old wonder had hardly changed at all in the last 100 years.

(China Daily February 1, 2007)