Miles of shifting sands surround the 30-foot-high (10-meter-high) ramparts of the fortress of Khara Khoto, or Black City, in northern China’s Alashan Plateau.

In the 14th century, China’s Ming armies laid siege to the Mongol city and diverted the Black River, which flowed just outside the fortress.

(Photo shot on assignment for, but not published in, “China’s Unknown Gobi: Alashan,” January 2002, National Geographic magazine)