To reach them a century later, I drove out of town some ten miles—past the modern airport, down a long and dusty road between melon patches, across a small stream—and came to where a low cliff began to rise beside the roadway. Before long, the cliff was a hundred feet and quite sheer, and there were holes incised in its face. Then I came to a fence and a guard, and had to pass through barriers and tollgates and pay a fee. It had been recommended that I hire an English-speaking guide, and I found one named Jessie. With her leading the way, I passed through another security check and then came to the face of the now almighty cliff.

There, incised into its sheer face, were the doors and openings of the caves that Stein had seen—cave after cave after man-made cave. Some were at ground level; others were up on terraces, accessible now by stairs; some were linked by ladders. Some were adorned with pagodalike frontages; some were swathed in fresh stucco with nasty modern iron doors; some sported prettier fretworked wooden doorways. Jessie had collected the keys to ten of them—if she carried keys to all five hundred, she joked, she'd collapse under the weight. "We'll visit eight today, perhaps ten," she told me. "And if you like, another ten tomorrow."

She turned the key in the lock on one unremarkable-looking door. Beyond the threshold it was dark, and then she opened the door wide, letting in the afternoon sunlight. I was quite unprepared: What spread before me was astonishing, overwhelming, the stuff of dreams. The walls were covered with hundreds, maybe thousands, of brilliantly colored images of the Buddha—all of them identical, all hand-painted or block-printed onto the wall no less than thirteen hundred years ago, and all so fresh that they looked as though they were completed yesterday. And then the roof: Every square inch was similarly covered with illustrations, all scenes—eighty-six of them, it turned out, and each different—showing the various stages in the life of the Buddha. Jessie explained that these were from the Northern Zhou dynasty, which flourished from A.D. 557 until 581—meaning that the painters who achieved such magnificence were working, so intricately and carefully, when Englishmen were still daubed in woad and America wasn't even a glint in a Genoese eye.

We spent the day going from cave to cave, fascinated. We never made it to cave 465, where there are said to be some wildly sexy tantric murals. But we went to others with enormous white-limed and colored statues of women and horses and Buddhas, and with images of men and women who looked distinctly Indian (and which had been created by artists so long ago that they saw Buddhism as a theology coming from below the Himalayas, and having little that was Chinese about it). In two of the more enormous caves, there were stupendous hundred-foot-tall Buddhas, so huge and splendid that one was bound to recall the great Buddha of Bamiyan in Afghanistan, which was shelled into oblivion by the Taliban seven years ago.