Amid Sichuan’s Hengduan Mountains, mysterious stone towers built jut more than 100 feet into the sky. No one is sure why.

Pengbuxi, Sichuan Province, China The 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.

Earlier this year, a teacher friend named Yang Wendou and I hiked across the Hengduan Mountains from south to north. We climbed through forests of fir, spruce, and Yunnan pine. We skidded down snowy passes within sight of Tibet. We breathed razor-cold air at 15,000 feet. We yo-yoed among ice peaks for 220 miles. We saw many wonders. Ours was perhaps the first foot traverse of the vast mountain range, an eastern extension of the Himalaya, undertaken in generations. But I can tell you this:

We discovered nothing new about the strange towers of Pengbuxi.

The colossal pillars stand mute sentinel over a remote alpine wilderness. Enigmas from another world. Dreamlike megaliths. They still hold all the power of a kept secret.

There are more than four towers.

In the past, scattered across the corrugated highlands of western China, there actually may have been hundreds. Only a few survive today in various states of preservation or decay. Some are reduced to mere piles of rubble. When the Austro-American explorer Joseph Rock passed through the Hengduan Mountains in 1929, for example, he observed “a conglomeration of tall leaning towers” near an old trading outpost called Jiulong. When Yang and I trekked through Jiulong, they were gone.

There was, however, a new highway tunnel.

The tunnel cored out almost two miles of solid rock near the summit of a 15,000-foot pass. The highway was closed because of heavy snows. The tunnel caretaker was named Shen Hao.

Shen Hao was from the ethnic Yi minority, a middle-aged father of three school-age girls, who spent weeks living alone, marooned in a smoke-blackened hut next to the tunnel generators. He cooked on a woodstove, carried icy creek water in plastic buckets, and hung garish hanks of yak meat from a ladder in the cold. He had the immobile face of a man who enjoyed solitude. He had married into the third richest family in Chengdu, he said. His sister-in-law was enrolled at Harvard. His father-in-law sometimes wired Shen Hao large sums of cash via the WeChat phone app. This paternal generosity was guaranteed because Shen Hao never once asked for it. He refused to work for family.

“I have everything I need. Things with price tags? You cannot take them with you when you die,” Shen Hao said, clouding his hut with woodsmoke as he prepared us butter tea and noodles. “Things without price tags, like love and friendship, maybe those you can take.”

The tunnel was like a tower laid on its side: A long, dark, frozen tube from which we emerged, blinking, into a winter landscape so bright it momentarily made me weightless, lifted me, like a snow crystal, off the ground.

Maybe if you burned diamonds you’d get light like that.

The Hengduan Mountains sprawl 560 miles long and 250 miles wide.

A product of the colliding Indian and Asian tectonic plates, the wild range knuckles up in parallel white scarps, each carved by plunging river valleys coursing north-south. (The ridges of the western Himalaya, by contrast, run east-west.) Because of this, the region stands out as one of the most biodiverse landscapes on the Earth. Elevation and latitude crosscut to form mazes of high-altitude grasslands, chilly black-green conifer forests, temperate rhododendron thickets, and subtropical lowland savannas. The human picture is hardly less scrambled.

Chinese ethnographers often refer to the mountains of southwestern China as a tribal corridor, a crossroads of antique migrations dating back to the Stone Age. Pastoralists such as eastern Tibetans (called Khampas) live there. So do members of minority groups such as the Yi and Pumi. And the descendants of Mongols and Han Chinese settlers. Human diversity muddies research into the origins of the Himalayan towers.

“Chinese experts consider that the towers have all been built by Qiang ren (Qiang tribes)” that once inhabited the fringes of the Tibetan Plateau, writes Frederique Darragon, a French cultural preservationist who carbon-dated wooden beams from several towers, producing age estimates spanning 1,200 to 800 years old. “There are many legends about the towers, none of them shedding much light on their possible raison d’etre.”

Darragon believes defense was the likeliest purpose of the extraordinary towers looming over Pengbuxi and elsewhere in the eastern Himalaya. (Firing slits for archers are carved into some of the structures.) But others hypothesize food storage, monuments marking male births, status symbols of the rich.

As for the Khampas prodding groaning yaks around the four giant pillars at Pengbuxi, they simply shrug and amble on.

Yang and I climbed to fairylands of snow-laden spruce and fir. We crossed yak pastures raked by winds. We burned under a devastating white sun.

Near the watery blue shadows of Mount Gongga, at 24,760 feet the tallest peak in Sichuan, we sheltered in the home of Sonam Zeren.

Like other ethnic Tibetan women of her generation in the Hengduan, Sonam Zeren had never set foot in a classroom. She could neither read nor write. Herding yaks at altitude, she and her husband had saved for decades to catapult their two boys into university. She still worked from sunup in her village, renting rooms in her house to traveling teachers and medics. It was hard to get a fix on Sonam Zeren. Her head came only to my shoulder. She was a tireless and cheerful blur. During our three days of recovery from exhaustion, mostly spent at her kitchen table, Yang and I rarely saw her sit down. When we finally set out on a bracing 12,000-foot morning, she sternly zipped my coat up to my chin. She packed us lunch.

Recently, an earthquake cracked Sonam Zeren’s mountain.

Neither of us speaks Mandarin. I texted her a sad-face emoji. Within seconds, three circular hand signals pinged back confidently: OK OK OK.

Hieroglyphics fit the Hengduan. Mutually incomprehensible languages pool in adjacent valleys. The range emanates rivulets of dialects. Its ridgelines divide accents.

The mountains are a tower. And their name could be Babel.

Professor Luo Xin, an expert in medieval Chinese history at Peking University in Beijing, does not know precisely why the baffling stone towers of the Himalaya were built.

Luo suspects their purpose may have changed across geography and time. But he informs me: “You can find them not just in the Hengduan Mountains but in Beijing.”

A Qing emperor built replicas of the remote Sichuanese towers in his capital, Luo says, to train his troops on siege techniques. This was in the 18th century.

“A few are still there,” he says, laughing. “Nobody really looks at them twice.”

We bunked down with four yak herders. Ice crystals blasted the side of their tin hut with a melancholic sigh. Outside, a bull lay frozen to the bone in the snow.

How did shepherds track their individual animals through the immense cosmos of the Hengduan Mountains?

“Oh, we know their faces,” Sonam Badeng explained. “We recognize them.”

Yang and I tottered over a snow pass where mineralized springs gushed water bright as arterial blood. We hit the first lowland roads near Jiagenbaxiang, and the houses of the settled Tibetans were like stone mansions. We’d missed the era of the last black tents by at least 15 years. A friendly pig—hogs ranged freely in numbers in the Hengduan Mountains—trotted up to have its ears scratched. It toppled onto its back, blissfully asleep, after less than a minute. At Pengbuxi, the towers stood like obelisks dropped from a ceramic blue sky.

“They built them to warn of bandit attacks,” said a local businessman named Dengzhu Zhaxi, who was taking his young daughter on a tour of the remote site. “Fire by night, and during the days smoke. Notice how all the towers are line of sight.”

Days later, limping beside the highway between Lhasa and Shanghai, I realized that wasn’t right.

The towers of Pengbuxi are styluses: They were scratching out the stories of our lives on the grand disc of sky that revolves, eternally, above the Hengduan Mountains of Sichuan.

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