Lake Rojo Aguado, the probable location of the pre-Colombian Moxos Crocs as big as cars would put anyone off taking a plunge. But Simon Chapman is willing to dip his toe in if Flipper's about civilisation, has always been a hard place to get to. An Inca army made it there from the Andes, but with its lines of supply overstretched it was wiped out.

The Spanish conquistadors failed too. Two competing expeditions got through the mountains, rainforest and swamps, then fought over which party was going to conquer the "savages" - who emerged from the jungle and dispatched those that were left. Even as recently as the early 1920s, when the German adventurer Leo Parcus reached here, the area was said to be impossible to get to, full of man-eating anacondas and black caymans. The cowboys of the pampa talked about a great lake, but none of them had been there.

Four days in, Lake Rojo Aguado seems just as unattainable to Julian and me.

Our first challenge is 18 bone-jarring hours by bus down the east side of the Andes from La Paz, dropping 4,000 metres to the jungle town of Rurrenabaque, with the local variant of techno-cumbia overloading the driver's undersized speakers.
After a few hours' sleep, we spend the next six on a jeep bouncing along the highway towards the Brazilian border. This time I think there is a flamenco tape in the cassette player, but it is only the jeep's suspension twanging like a Spanish guitar.

By nightfall we are hanging our mosquito nets in a house next to the tiny Rio Yata, watching the crimson full moon rise above the savannah. We feel ready for anything: we've bought machetes in the market in La Paz, and all the bits of our canoe have arrived safely on the plane with us.

Now, we chug down the Rio Yata in a 40ft dugout launch powered by a two-stroke motor called a teke-teke because that's the noise it makes. It is incredibly slow. I clock it at 6mph with the speed function of the GPS.

It is only 30 miles to the spot where we will cross the savannah to the lake, but with the meanders and the teke-teke we'll be lucky to make it in three days.

We are enclosed by high mud banks topped with scrubby jungle vegetation. It is rather like we are going down a twisting, meandering water-filled trench. Kingfishers and herons constantly fly ahead of us, and red-eyed hoatzins that look like something out of the Jurassic era hiss and cough as we go past. Capybaras, like sheep-sized guinea pigs, duck through the water or galumph up the banks away from us and, more disturbingly, crocodiles lurch into the water as we approach. While I am writing this we pull alongside a dead croc two and a half metres long. Irgen, our guide, says its jaws could crush a cow's skull. This black cayman is only half-grown.

It isn't just me and Julian; even Irgen and the other Bolivians with us won't go in the water to wash - except, that is, when the dolphins are around. Amazon river dolphins are pink with long bulbous heads through which they focus the high-frequency sound waves that they use to navigate the turbid water. When dolphins are around, it means piranhas and caymans are not. When you're looking for an unattainable lake in the middle of the jungle it's good to know at least someone is on your side.

Simon Chapman is head of physics at Morecambe high school and author of the Explorers Wanted! series for children (Egmont) and The Monster of the Madidi (Aurum Press). Follow his progress for the next five weeks in The TES and keep up with the Knowsley project at www.spiritofthejaguar.org.uk