Hot Spot: Inner Mongolia

Our guide tries to explain that the spot we're heading for in this godforsaken corner of the Gobi Desert, not far from Ejin Qi in northwest Inner Mongolia, was once a bustling urban center—an important marketplace for Mongolian traders from the north and Uighurs from the west and south.

But after driving several hours along a trackless route visible only to locals, I am a little dubious. Then suddenly on the horizon we see the still-massive walls of the so-called Black City, evidence that this inaccessible nowhere was, long ago, a hub of commerce. The fort surrounding the long-abandoned town was built in 1036 as a military outpost of China's fabled Xia dynasty. Elegantly pointed archways and windows punctuate the half-kilometer-long stone walls; massive onion domes, though crumbling, still ornament the fort's corners.

This was once a thriving oasis, our guide explains: the Black River fed the ancient city until rival troops laid siege in 1322, cutting off the water supply and forcing inhabitants to surrender. Now the river has dried up and the Black City, once home to thousands, stands as a cautionary tale of ecological vulnerability, the walls in some places barely taller than the massive sand dunes that lap at the gates.

The desertification process—productive areas turned into wasteland—is now happening across Inner Mongolia due to more modern threats: farms and factories are diverting increasing amounts of water from overburdened rivers in the rush to expand production and industrialize. And lost in the silence of the desert sands, the Black City's walls now protect only pottery shards and the bleached bones of goats and camels.