Nobody knows how many died during the building of the Karakoram Highway. The road is only 750 miles long, but passes through some of the most imposing scenery on Earth.
Twenty years were needed to lay the thin strip of asphalt that now connects Kashgar, in the extreme west of China, with Islamabad, the capital city of Pakistan.
Over those 20 years, Pakistan continuously employed more than 15,000 soldiers to complete the task. Mountain landslides, accidents and brutal climatic conditions killed at least 500. China, said to have utilised vast numbers of prison convicts, has never revealed its share of the death toll. Completed in 1986, the Karakoram Highway immediately became one of the great road trips.
Its beginnings are low-key. Leaving the traffic fug of Kashgar behind, it takes a day-and-a-half to cycle across a dry and dusty desert plain, before finally arriving at the police checkpoint beneath Ghezchun Canyon.
With passports required, it feels like the entrance to a different country, although Pakistan is still 200 miles beyond. After the checkpoint, the highway begins to climb and the landscape transforms.
Soon, sharp winds are scouring the face of Mt Kongur and racing down into the canyon, whose floor is littered by immense boulders deposited here during the last ice age.
Mt Kongur (7,719m) is just the first peak. Over the next fortnight, the highway will steer a course through the largest concentration of 7,000 metre-high mountains anywhere in the world.
Temperatures also change dramatically. Three pairs of extra-thick cotton socks, worn at the same time, are no longer enough. Heavy snowfalls often close the highway from mid-November onwards. But today it's too cold to snow.
The infrequent settlements that border the highway are made of mud-brick and populated by ethnic Kazakhs and Tajiks, as opposed to Chinese. Rugged people, their livelihoods are tied to herding yaks and goats. Cattle are unable to survive at such high altitudes.
After another day cycling, the canyon slowly widens. Mists hover eerily above the salt flats of Bulan Kul, where the scenery is so bleached of colour it looks painted and unreal. The day draws to an end in sight of the turquoise waters of Kara Kul. The lake is a summer beauty spot, but the season is long finished. A dingy-looking hotel stands by the waterside, locked and boarded-up.
I rest for the night in a Kazakh dwelling, drinking tea flavoured with salty yak milk, sleeping beneath heavy woollen rugs, kept warm by a stove that burns yak dung.
It's a way of life that is almost unchanged for countless generations. Almost. As he shepherds the family flock back into the compound, the eldest son is riding a motorbike. While she prepares noodle soup, the eldest daughter is eagerly watching a fuzzy black and white television set. China's pop idol contest is showing tonight.
Pinned to the wall behind the TV screen, a pair of posters: Deng Xiaoping, Mao, and a few more recent leaders are portrayed in one; Mecca is the subject of the other.
Dawn, before the headwinds stir. The trudge begins up to Ulugh Rabat, a 4,000 metre-high pass through the mountains. It's an effort... but what a ride down that afternoon.
15 miles without pedalling; freewheeling as the snow falls thickly; feet both turning worryingly numb. Then civilisation moves to reassert itself again. The roadside stall sells little but out-of-date crisps and sickly-sweet Chinese chocolate bars, but after four days of nothing except noodles, this limited selection resembles the food stall at Harrods.
In China, you're never too far away from being told what to do. The town of Tashkurgan marks the end of the Karakoram Highway for a while. Or at least it does for foreigners, who for "security reasons" must now travel by bus the further 100 miles to the border.
Tashkurgan's best-known hotel is opposite a supermarket equipped with a tannoy system. A woman's voice blared out at full volume. Once she would have been spouting communist propaganda, now she was advertising low, low prices.
The hotel's dormitory room was unheated, cheap, entirely empty and next to a lavatory so spectacularly filthy that it took your breath away (if that's the right expression). A much more welcome discovery elsewhere in the hotel was Emilio and Maria, a pair of Spanish cyclists I had met several weeks before in Kyrgyzstan.
We went to eat together, and over a fine Cantonese meal, I learned that three other familiar cyclists – Alex, Chris and Larissa, from Holland – were just a day ahead, having caught the bus that morning.
Tomorrow we would set about trying to catch up with them as the Karakoram Highway continued into Pakistan and the incomparably beautiful Hunza Valley.