Grand tours: Somewhere under this bush is El Dorado

Writers and their adventures in literature: John Blashford-Snell and Richard Snailham explore the Bolivian Amazon

Born in 1936, John Blashford-Snell, one of the world's most respected explorers, studied at Victoria College, Jersey, before entering The Royal Military Academy, Sandhurst. Richard Snailham, formerly foreign secretary of the Royal Geographical Society, joined him on a 2001 expedition in the Bolivian Amazon, in search of Paititi, the mythical land the Spanish conquistadors called El Dorado. This is an extract from their book 'East to Amazon'.

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As the day grew hotter, tiny monkeys feeding on the blossoms above us chattered in alarm as we passed, and the great electric-blue butterflies flitted ahead of our column. Some of the porters swigged alcohol, and they all chewed coca incessantly to give them strength - or to perhaps numb the pain of their loads.

As we reached the ridge the trail flattened out. The walls of the "fort" were now exposed, and appeared to be much better constructed than those at Tulani. Faced stone had been used, in the Inca tradition. "Inca," said Juan Domingo confidently, showing me some pottery. "I think it's an inn, not a fort - probably used by traders travelling up from the river." Ertl had talked of another ruin 200 yards away, but, though we searched all day, the dense jungle and bamboo obscured everything. Hidden from view, 2,000 feet below, on the Rio Chinijo, Craig Halford's bridge was progressing slowly. Sam Allen, who had earlier joined us from advanced base, now provided medical cover for the main party. I was still worried about having a serious injury in this treacherous terrain. Sam was an asset in many ways. He is an excellent cook, and his culinary efforts did much to raise spirits. Paul Overton had joined us again, looking rather old behind his shaggy grey beard. However, it gave him a measure of protection from the wretched bees that had returned to the attack. We all got stung and, remembering a near-fatality from a hornet sting on the Darién expedition in 1972, I hoped we had no one with an allergy to them.

Then the rains struck. The descent to the Chinijo became more than normally hazardous. We slid downhill, ducking under low trees and clambering over massive trunks which blocked the way. Vines snapped and sent us slithering uncontrollably. "If we get everyone down this cliff without a broken leg," I said to Yoli, "it will be a miracle." But we did. Near the bottom the track was supported by ancient walls of faced stone, and there were terrace walls and broad platforms. Everywhere one looked there were signs of an earlier civilisation - but the two Bolivian archaeologists were too far sunk in misery to notice.

Craig Halford's engineers had worked hard on the bridge. A 60-foot length had been cut from a tree felled nearby. It didn't quite reach the far bank, so they had built a pier of stones for it to rest on. On top of it they had nailed short slats of wood to make a safer walkway, and had rigged a cord handrail. But crossing it would still be perilous: the Chinijo surged underneath, and good balance was required.

In pouring rain I said, "I name this bridge Halford's Bridge", and with my machete I sliced through a coloured ribbon tied across by Dr Jo Brown. Juan Domingo then donned a life-jacket and nervously made his way over. To have fallen off with a heavy pack would have been to invite drowning. "Don't look down at the water" I cautioned Jo as I set off. That, of course, was an invitation to do so, but I nevertheless made it across. The ensuing wet season would probably see the giant log borne away on the flood, but at least I could get our expedition over it.

'East to Amazon', by John Blashford-Snell and Richard Snailham (£8.99, John Murray). Readers of the 'Independent on Sunday' can order a copy for the discounted price of £7.99 (including p&p within the UK). Call 0870 121 0009 and quote BSH051.