Fishermen's tales the world over have a reputation for being unreliable, and those you hear in Amazonia, as might be expected from the largest body of fresh water on earth, are more far-fetched than most.

In June 1995, while grilling Amazon fishermen about something else entirely (an animal I had photographed but which, it appeared, nobody else had seen!), I heard two stories that were outrageous even by Amazon standards. But I was struck by one odd detail that both had in common.

Dorgival Sabino, described what he saw in the Rio Negro near the city of Manaus thus:It was a gigantic animal, like a monster. A snake, but of a size much bigger than normal, with the difference that its head was like some kind of dinosaur, with -- I don't know whether they were teeth or horns, just that it was grotesque. Its length, he guessed, was over 20 metres, and its diameter approaching a metre.

I had heard about the Boiúna or Cobra-Grande, the giant snake of Amazon myth. But I had not previously met anybody who claimed to have seen one. Believed by some to be a outsize anaconda (Eunectes murinus), by a few to be another, as yet unknown, species, it is generally dismissed by the outside world as a creature of the imagination. This remained my view until, 300 miles away, up a minor tributary of the Rio Purus, I met Amarilho Vincente de Oliveira.

A down-to-earth 55-year-old, accustomed to navigating these backwaters day and night, he saw something in his torch beam one night 20 years ago that at first he thought was a floating tree trunk. Looking back moments later, he was alarmed to see that it had turned 90 degrees to face him, despite there being no current. He and his companion hastily doubled back the way they had come, crossed the 100-metre-wide channel, then stealthily paddled up the other side ­ only to find it facing them again.Its head had horns like the roots of a tree, and we could see these greenish eyes as well," he recalled. "We just left the canoe on the bank and got out of there... Afterwards, people saw movements in the water there, many times. With no wind, there would be waves that covered the beach.

A snake with horns? It sounded unlikely. But that two people should make up the same odd detail independently seemed equally improbable. Also, none of the historical accounts of alleged sightings mentioned this.