The wooden platform looked out onto a teardrop-shaped lake a mile across, set in the centre of thick jungle. Siobhan dived in first and swam towards the middle. I followed her into the green velvety water. At the approach of dusk - with a troupe of spider monkeys swinging and chattering through the trees on the far bank for a final feed before bed, and a pair of green and scarlet macaws squawking overhead - we trod water in the centre of the lake, listening and watching as clouds began to darken the pink and gold sky. Suddenly the clouds burst, and a million pearls of rain were bouncing off the surface. Then, as suddenly as it had started, the rainstorm finished. In Chalalan, in the middle of the Bolivian rainforest, we had a sense of paradise.
A week earlier, 400 miles to the south, we were in Potosi - a town full of ghosts and, according to one Spanish chronicler, site of 'the mouth of hell'. At 4,000 metres above sea level, Potosi is claimed to be the highest city in the world. With its 2,000 colonial-era buildings, Potosi is a Unesco world heritage site, but the beauty rests on a foundation of tragedy. Towering over the town is the Cerro Rico (rich mountain) - a dusty pink cone, once full of silver, now riddled with half a millennium's worth of miners' tunnels.
Spanish conquistadores began mining silver in 1546, forcing the local Quechuans into the mountain to extract it. Over the subsequent 500 years hundreds of thousands died to provide the Spanish crown with silver. While the village turned into one of the world's largest cities - vying with London, Paris and Seville to be the richest - one estimate has Cerro Rico swallowing eight million Bolivians. The two billion ounces of silver that the mountain held long ago fled to Europe.
What is left to Potosi is 32 ornate baroque churches, a fortress-like royal mint - where the silver was stamped into the pieces of two, four and eight so prized by the pirates of the Caribbean - an impoverished population, and a nascent tourist industry that finally takes Europeans into the jaws of Cerro Rico.
'Do we travellers want danger or not?' a companion on my mine tour asked rhetorically. 'I did this morning, but now I'm not so sure,' someone replied. Having signed a waiver form clearing the company of responsibility for our safety, we were climbing into wellies, overalls and miners' helmets in a house on the edge of town. Then it's off to the miners' market where we buy 'gifts' of soft drinks, coca leaves and sticks of dynamite for the miners. As we plod into the black opening of the mine, I try to distract my burgeoning claustrophobia by stuffing coca leaves into my mouth and chewing intently. Everyone is more or less terrified by the desperate conditions - the stifling crawl through tunnels, the rotting struts overhead, the noxious clouds of gas fouling the already fetid air - but our fear just serves to remind us how lucky we are to be spending just a few hours underground, rather than 20 years. It's only when we get out that our guide tells us Cerro Rico's death toll is still 25 miners a year. Stunned by relief at being in the open air, as our guides light sticks of dynamite we stand grinning for the cameras with them fizzing in our hands, before they run off to place them at a safe distance to explode.
Bolivia, at the heart of South America, boasts more strange attractions than any country I've visited. The vast breathtaking and eerie salt flats of Uyuni; the solemn, sacred azure Lake Titicaca, where the Inca civilisation was born; the capital La Paz with its skyscrapers, Coca Museum, and the witches' market of llama foetuses and love potions.
Sucre is a beautiful, sunny city of white-brick colonial buildings nestling in a bowl of hills 10,000 feet up in the Andes. Although high, after Potosi the air is joyfully oxygenated. Sitting at a table on Cafe Mirador's terrace we drank strawberry and orange juice, one after the other, for hours. Emerald hummingbirds zipped from plant to plant, and occasionally we cast our eyes down over the terracotta roofs of this Andean Siena.
In any visit to Bolivia you are bound to bump into a fiesta sooner or later. We hit the jackpot in Sucre with the Independence Day festival. Brass bands, parades, concerts, fireworks, feasting and dancing: even a visit from the popular new President Evo Morales, the first indigenous leader in South America, whose radical reforms have created a government actually working for the people.
Potosi, Sucre and Chalalan exemplify the different sides of the country - the arid western half perches high in the Andes, before plunging spectacularly down into the tropical jungle of the north and fertile plains of the east. And what better way to appreciate the highs and lows of Bolivia than to cycle down 'the world's most dangerous road' - a dubious boast, as anyone who has been in Iraq recently will surely testify. Starting from the frigid, arid heights near La Paz, the three-hour journey takes you through the fertile semi-tropical Yungas into the humid tropical Coroico. With drops of almost a kilometre off the edge of the thin dusty ledge, it's a spectacular freewheel through three completely different climates and 'ecosystems'.
Down a less steep road from Coroico is Rurrenebaque, a town of stilted wooden buildings sweltering on the banks of the Beni river. From here we take a six-hour ride in a motorised dugout canoe into the heart of the Madidi National Park: two million hectares of pristine rainforest, home to the greatest biological diversity in South America. We chug through vast gorges into thick jungle, with a whooping soundtrack of Primal Scream on the iPod and herons, kingfishers, turtles and caimans providing the visuals. After a kilometre-long hike through jungle, we reach the seven lakeside wooden cabins that make up Chalalan Lodge.
A tribe of Quetchua-Tacana Indians from the nearby village of San Jose decided to set up a lodge where they could guide the tourists around their native forest in an ecologically sound way. Chalalan is the exceptional result - one of the most beautiful spots on the planet, and widely regarded as one of its finest examples of indigenous eco-tourism.
All day and night the jungle is full of exotic noises: the grunting of prehistoric watsons - turkey-like birds sounding like a herd of emphysemic pigs; the basso profundo of the howler monkey, sounding like a distant motorway; and the king of them all, the tiny Lorenz's thrush, which can mimic over 40 different types of bird but, rather tragically, has no call of its own.
Our extraordinary guide, Alejandro Alvarez, turns walks along the rainforest trails into our very own series of Planet Earth. As we walk past vast buttress-rooted mahogany trees and bizarre walking palms, he explains what the plants are, what their role is in the complex jungle ecosystem, and what uses - as weapons, foodstuffs, building materials, medicines - his tribe have for it. As we return to the lodge, we pick juicy pink grapefruit from a vast, heavily laden tree. 'A guest dropped some pips on the floor five years ago,' Alejandro tells us.
That night we take a canoe onto the lake at night and shine our torches, lighting up the red eyes of the caiman resting in the fringes. 'Caiman?' Siobhan asks. 'Oh, yes,' laughs Alejandro, 'only sevenfeet-long, not big enough to trouble you. There are piranhas as well.'
Tomorrow's swim has an added frisson. But this is Bolivia, full of extraordinary experiences. And if some are heavenly, some hellish, all are magical.
Essentials
Tom Templeton travelled with Journey Latin America (020 8622 8491). A tailor-made tour of the highlights of Bolivia, finishing in Chile with the Atacama desert and the capital Santiago, costs from £2,471 per person (twin share basis) for 17 nights. In Bolivia, the itinerary takes in La Paz, Sucre, Tarabuco, Potosi, Uyuni and three nights at Chalalan Lodge. International flights are included.