The Yungas Road is a one-lane dirt shelf northeast of La Paz that falls 3,500 metres from the Andean passes to the edge of the Bolivian jungle, and nothing much uses it any more. Paraguayan prisoners of war cut it into the mountainside in the 1930s, during the Chaco War, working with picks, shovels and dynamite and almost no engineering at all. That is why it stayed the way it is: a ledge barely three metres wide, blasted out of near-vertical rock, with no guardrail on the drop side and no room for two vehicles to pass in comfort. It also has a rule found nowhere else in the country. Bolivia drives on the right, but here the descending vehicle keeps left, on the outer edge, so that the driver can lean out and watch their own wheels against the lip while something coming uphill squeezes past. For decades this was the only link between the capital and the Yungas valleys. Then a paved road opened in 2006, the trucks and buses moved onto it, and the old track was left with nothing to carry.
What it carries now is bicycles, about twenty-five thousand riders a year. You start at the La Cumbre pass at around 4,650 metres, where the air is thin and the altiplano is bare and grey. The first twenty kilometres are asphalt; near Unduavi the old gravel takes over and the track narrows. From there it is all downhill through one climate after another, tussock giving way to bamboo and bamboo to jungle as the air thickens and warms around you. Lower down, thin waterfalls come off the cliff and run straight across the dirt, so you ride the last hours through spray and slick mud. At the hairpin called the Corner of Death, the sharpest bend on the descent, you get off and walk to where the dirt simply ends in open air. The morning finishes in Coroico, 1,200 metres up but tropical, in heat and banana green, with the mountain gone into cloud behind and a road on it that is no longer going anywhere.