Pengbuxi, Sichuan Province, ChinaThe stone towers of Pengbuxi, a hamlet of yak herders and barley farmers that pools in an 11,000-foot-high valley in the Hengduan Mountains of southwestern China, jut into the sky like colossal exclamation marks.

These towers—four survive, though villagers say there used to be more—are a marvel of ancient engineering. Their spires rise as high as a hundred feet above the surrounding fields: a remarkable stature for any freestanding structure built by hand, from gray, unworked rocks. They are eight-sided, star-shape in cross-section. They taper elegantly toward their tops. How old are they? What was their purpose? Why are they even here? Chinese historians still debate these questions. The people who erected them remain largely unknown and left few written records.

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