A Mongolian map of the Omnogovi region traversed by the Wall of Genghis Khan.
Known locally as "Wall of Genghis Khan", the Wall was constructed from blocks of basalt (top), thick saksaul branches and sand (middle and below). It is about 40 km north of the China-Mongolian border. James and William Lindesay / For China Daily
Saksaul used in the Wall of Genghis Khan was found growing in groves across the south Gobi region of Mongolia. James Lindesay / For China Daily
Beijing-based Briton William Lindesay led an expedition into Mongolia's Gobi Desert in search of 'The Wall of Genghis Khan' to make one of the greatest discoveries of his 25-year-long exploration and research career at the Wall.
What is the "Wall of Genghis Khan"?
After 10 hours of driving across the empty steppe, we reached Dalandzadgad, capital of Omnogovi, or the "South Gobi" province of Mongolia, where our expedition began.
Although the Gobi is notoriously heartless, ahead lay the heart of the Gobi - its widest tract.
Before taking the leap, we took precautions in loading up to the brink.
At the gas station, our tanks and two spare canisters per vehicle, gave each a 600-km range - double our estimated 300-km route.
Next came water, also from a station, sold by the bucket.
Finally, we called at the drinks store, for 160 liters of "Mirage of Gobi" mineral water.
We also procured liquid gifts for the army. We'd need their help to find what we were looking for - appropriately, we chose a few bottles of Genghis Khan vodka, because we would be meeting officers from the Mongolian Army and looking for what the Mongolians call "The Wall of Genghis Khan".
An atlas had put Mongolia "outside the Wall" at the top of my Great Wall research agenda.
All its maps featured a swerving red arrow, showing the journeys of one man.
Page after page, year after year, this arrow recorded hit-and-run raids for horses and women, attacks to subjugate surrounding tribes, sustained wars against arch enemies and protracted campaigns to topple kingdoms and empires.
All journeys occurred between 1162, when a boy named Temujin was born, and 1227, when "The Universal Ruler" as he'd risen to become, died.
Entitled The Genghis Khan Atlas, this geographical version of The Secret History of the Mongol told the life story of Genghis Khan, and it had seized my curiosity because it depicted walls throughout various parts of Mongolia. All of them were labeled: "The Wall of Genghis Khan".
Did the great man authorize the building of long walls?
How and why could a nomadic warrior be a conqueror and a defender?
And, if he had built walls, who was he attempting to fend off? Or, thinking "beyond the Wall" and its conventional defense uses, were they built for other purposes?
To simplify the geography of these mysterious structures, I marked them on a sketch map of Mongolia, in an attempt to work out if they bore any obvious cross-border relationships with the Great Wall in China.
Historical atlases of China showed that three dynasties - the Western Han (206 BC-AD 24), Liao (916-1125) and Jin (1115-1234) - had routed sections of Great Walls across land that today straddles the China-Mongolia border.
It was the Han structure, built 21 centuries ago, that aroused my strongest interest, as I had researched its finely preserved remains extensively at its western extremity in the vicinity of Yumenguan, or the Jade Gate Pass, in Gansu province.
By what I soon dubbed as the "jigsaw" theory, it seemed likely that the "Wall of Genghis Khan" in Mongolia's Omnogovi province might be part of the Han Great Wall, marooned outside today's China by the change in border locations over the millennia. However, I wanted to collect direct evidence with fieldwork, allowing comparative studies to have a say in consolidating that theory.
Although access to a sensitive no-go border region seemed impossible, I continued my research by asking questions.
Fortunately, a Dutch friend, Tjalling Halbertsma, an old Wall exploring companion from the late 1990s, had based himself in Mongolia's capital, Ulaanbaatar.
He e-mailed me a photo of the Genghis Khan Wall.
However, the image that I received showed a Mongolian accompanied by his wife and child on a motorbike.
I replied, advising he'd sent the wrong picture.
"But it's the right photo," he insisted. "The Genghis Wall is also called the Genghis Road since it's exploited as a landmark to lead people safely across the otherwise empty landscape."
Halbertsma made a real breakthrough years later, in 2011, finding Professor Baasan, an eminent geographer and specialist on desertification who was taking a growing interest himself in the origins and purposes of Mongolia's Walls.
Both gentlemen were prime movers in swinging the high-level government approval for the first international research expedition into the prohibited region.
With the last oasis of Dalandzadgad now behind us, Baasan sat upfront to navigate our Land Cruisers' cross-country passage across the Gobi.
With 50 years' desert experience, he looked at his map, took in the contours, glanced at his GPS, predicted the presence of sand dunes and gradient of inclines, and duly instructed his driver on the best way forward.
We were heading toward a precise destination.
My eldest son James, an emerging Great Wall enthusiast, and more comfortable with technology than myself, had used Google Earth to follow the Wall of Genghis Khan across Omnogovi province.
On his aerial traverse he had identified very occasional dark shadows underscoring the Wall's otherwise faint line. These, we considered, might indicate where the Wall stood higher, where it might be better preserved.
One of the most promising coordinates was fed into our GPS devices, and this also became our agreed rendezvous point with the Mongolian Army at 4 pm the next day.
Our campfire conversation that night tilted toward the question of how nomadic invaders from the northern steppe had negotiated this immense, intimidating Gobi, a natural barrier between themselves and China, the land of plenty, and why anyone would have built a wall here.
Prior to our expedition's departure from Ulaanbaatar, I'd met Professor Jack Weatherford, author of Genghis Khan and the Making of the Modern World. He explained that the building of any restraining structure such as a wall would have been considered an offense against divine law in Mongol culture, as they respected the flow and cycle of nature.
"That is why Ogodei, Genghis Khan's third son and chosen successor, confessed that his building of 'tamped walls and fences' to prevent the migration of wild beasts off his land was one of his four great misdoings - according to The Secret History of the Mongols."
The next morning, a herd of about 50 gazelles (Procapra gutturosa) easily outpaced us as we sped at 70 km per hour across a saltpan.
Gazelles were a valued resource of meat, skin and horn for the Mongols.
Batmunkh, one of only two bow-makers crafting the traditional Mongolian bow in the country today, had told me the accurate range of the composite birch and ibex horn bow was around 70 meters.
In that context, a barrier constructed to block the gazelles' path may have provided a hunting advantage.
Could the Wall of Genghis Khan have served as a platform for archers to await the approach of herds driven toward them by chasers?
The lead Land Cruiser stopped. Baasan got out and approached. "It's the Genghis Road!" he announced.
And it was here that we met Captain Enkbayer, a portly army officer, who checked our papers from Ulaanbaatar's ministries and then led us over the rise. There stood the Wall of Genghis Khan, several meters high.
Saksaul wood made up about 30 percent of its fabric, and the branches were highly variable in size, ranging from spindly twigs less than a centimeter thick to long, thick branches, and occasionally, logs.
I recovered two samples of wood and a tiny strand of braided rope from the structure, hoping that radiocarbon dating tests on my return to Beijing might enlighten me through scientific means as to the era when this Wall was built.
The structure was remarkably like the sections of reed- and tamarisk-built Han Great Wall around Yumenguan.
There was one glaring difference, however. The Wall of Genghis Khan was absent of watchtowers dotted along its line.
Since Yumenguan's towers - originally made of adobe bricks and built to great heights and thicknesses - constitute the best preserved components of the defense system there, I had to conclude that they had been omitted from the construction plan here - for one reason or another.
According to a leading Sinologist on the Han Dynasty, Michael Loewe, writing in The Campaigns of Han Wudi, the "North Desert Offensive" launched by Emperor Han Wudi in 119 BC against the Xiongnu nomads lasted six months and brought victory after a long cross-Gobi and steppe chase but at an astoundingly high cost.
Tens of thousands of Han cavalrymen perished, along with 100,000 horses. Huge payouts of gold were made to the victorious soldiers. It busted the national coffers.
It seemed possible that the Wall here was built to consolidate that military victory, but the cost of finishing it off, replete with watchtowers, might perhaps have proved to be problematic to a government in deficit and futile.
Unlike the commercially important Hexi Corridor out west, which was evolving into the Silk Road with prospering oasis towns, the land here was useless. There was no water, and no trade route. There was nothing to defend.
Could it be that, soon after its initial phase of construction, the Wall project through the Gobi's heart was deemed too heavy a financial burden, so they abandoned it?
The next day, we maintained the position of our camp, offloaded all unnecessary equipment and set off west, crammed into one empty vehicle, to economize on fuel.
Captain Enkhbayer's gestures predicted our route. Interpreting his hand and arm language suggested the Wall crossed the desert in a straight line and made what appeared to be an abrupt detour over a mountain.
At first we bounced along, stalking the Genghis Wall, a feeble scar at times, improving in height to a higher mound, sometimes strewn with rocks.
Then, as indicated by a drop in speed, clashing of shoulders and ribs among passengers, and the revving of the wheels in loose sand, we were "off road". Within minutes, the wide blue sky turned milky and then brown. A dust storm was blowing in.
Sandak, our driver remained unruffled and persistent. We reversed, turned and backtracked.
Slowly, and by sheer instinct, he wriggled us out of this siege of sand and sky.
Eventually, we reached the only place name on Baasan's map: "Khermiin Undur Uul" or "High Mountain with Wall", which is actually an extinct volcanic cone.
The storm had passed us and was en route south, toward China's Ningxia Hui autonomous region. In the calm clarity, a structure made of dark, angular blocks of basalt appeared like a petrified snake, as if it had been created by the solidification of a torrent of turbulent lava flowing from the summit vent. However, it was neither geological nor reptilian in origin but, rather, military.
Striding up along its side, with expanding vistas across the desert as I gained height, it was clear that the builders made a distinct effort to route the Wall up and over this mountain for one purpose: to command the most supreme vantage points.
At the summit, I surveyed the scene through my binoculars. In this 360-degree panorama, with improving visibility, I estimated that I could see a radius of 30 km, and a quick pi calculation informed me that from here the observer could survey an area of 2,800 square kilometers.
Astounding was my realization that the Wall below was the only manmade object that had ever dared to challenge nature here.
If Han guards had observed the approach of enemy Xiongnu warriors from this mountain lookout 21 centuries ago, might Ogodei's hunters have used this same viewpoint after the Wall was rebuilt some 12 centuries later, to look out for the dust kicked up by herds of racing gazelle?
The wind began to stir, and visibility once again started to fade. Shockingly, my radius of observation shrank to just a few kilometers within as many minutes.
We tethered our hats, gathered together, snapping some last shots in defiance of the dust-laden gusts and started our cautious decent over the sharp rocks to our Land Cruiser, a barely visible speck on the hazy plain below.