With the help of Google Earth, an international expedition documented the ancient wall for roughly 100 kilometers (62 miles) in a restricted border zone in southern Mongolia in August 2011.
The defensive barrier formed part of the Great Wall system built by successive Chinese dynasties to repel Mongol invaders from the north, according to findings published in the March issue of the Chinese edition of National Geographic magazine. (The National Geographic Society is responsible for both the magazine and National Geographic News.)
Preserved to a height of 9 feet (2.75 meters) in places, the desert discovery belongs to a sequence of remnant walls in Mongolia collectively known as the Wall of Genghis Khan, said expedition leader and Great Wall researcher William Lindesay.
Named after the founder of the Mongol Empire, the Wall of Genghis Khan usually survives only as "a faint trace," Lindesay said in an email.
But "we found a 'real wall', standing high and existing as a dominant landscape feature," he said.
What's more, it wasn't the work of Genghis Khan or his heirs but actually a long-lost segment of the Great Wall of China network, the team's findings suggest.
First to Investigate New Great Wall?
Close to China in the border region of Ömnögovi Province, the ancient structure hadn't been scientifically explored or studied before, said Lindesay, director of the International Friends of the Great Wall conservation group, based in Beijing, China.
"We're the first to investigate the ruins," he said.
"According to the army officers who minded us, we were the first outsiders to be allowed into the area," Lindesay added. "We assumed various local Mongolians had been to the area, but had not considered the structure of much interest."
At times seeking out topographic clues seen in Google Earth—the wall is visible on satellite images—the team located two well preserved but contrasting stretches of wall.
One section had been made mainly with wet mud and a woody desert shrub called saxaul, the other from blocks of black volcanic rock.
Along its vast length, Lindesay suspects, the wall originally stood at least 2 meters (6.5 feet) taller than it does today.
"What we found was just the last remaining piece of a 'fossil'—the skull or the large thighbone, with the rest missing," he said.
"One can expect the wall was both much higher and continuous for vast distances."
That dark basaltic rock seems to have been an obvious choice for the second stretch, which crosses the rugged remains of extinct volcanoes.
The clean, straight edges to the blocks indicate that the stone was quarried, which would have required a large, organized workforce and an efficient transport system, the team said.
Ancient Mongolian texts suggest that the so-called Wall of Genghis Khan was built as an animal fence by Khan's son Ögedei to keep wild gazelle on his land. (Learn about National Geographic's Valley of the Khans project.)
But the recently explored Gobi Desert wall segment isn't in a region where large herds of gazelle occur.
Chinese researchers, perhaps not surprisingly, have speculated that China's Han dynasty had erected these little-studied stretches in about 115 B.C.
But radiocarbon dating of partly exposed wood and rope remains extracted from the wall indicates that the saxaul-segment construction went on for more than a hundred years—and occurred about a thousand years later than thought, from A.D. 1040 to 1160.
Those dates hint that the Western Xia dynasty built the walls—or at least rebuilt old Han walls on the sites.
Holding Back the Mongol Tide
This northwestern Chinese dynasty isn't known to have contributed to the Great Wall system, but in at least one aspect, a Western Xia origin makes sense.
During the Western Xia period, Mongol tribes were rising in strength and making forays south, Lindesay noted.
"If one imagines the wall as a platform, with some kind of battlement—perhaps of wooden stakes, functioning as a shield to those manning its top—then it would have been an effective defense installation," he said.
But, mysteriously, the expedition team found no pottery, no trash, no coins, no weapons—nothing to prove the wall was ever actually manned. Nor did they find any of the watchtowers that mark surviving sections of the Great Wall within China.
"The wall system was incomplete," Lindesay said. "It not only lacked the signaling capability [to make smoke signals]—it didn't appear to be capable of accommodating troops."
"I believe the wall here is only half built and that there was, for some reason, a rethink on locating the wall here," Lindesay said.
It isn't difficult to imagine how the purported Great Wall segment's harsh desert location might have led to the remote frontier defense being abandoned, he added.
Weatherford, the Minnesota-based anthropologist, agrees with Lindesay's conclusion that the newfound remains were Chinese constructions.
There's a good reason, Weatherford added, that the stretch nevertheless carries Genghis Khan's name.
Mongolians, he said, are sensitive to the idea of "Chinese structures built on their land, since it carries the possible claim that the land was once Chinese.
"By calling it the Genghis Khan Wall, the name makes the place Mongolian and rejects foreign influence," Weatherford said.
He also describes the expedition new findings as "very important, because to my knowledge this wall has not been studied."
"I would risk saying that it is the largest human-made structure or artifact in all of Mongolia," he added. "It is amazing to me that it is not already much better analyzed."