In Tintin's last-but-one-adventure, Flight 714, our intrepid boy reporter and his friends find themselves on a small and deserted Indonesian island. The first intimation that things may not quite be as they seem comes when a rather bemused-looking but undeniably giant lizard ambles out of the bushes.

"What on earth's that?" exclaims Captain Haddock. "A monitor," replies Tintin. "What's it doing here, pestilential pachyderm?" demands Haddock, displaying his usual grasp of alliteration (and absence of scientific knowledge). "It looks like it's escaped from the ice age!"

The alert reader will have spotted several errors here. This reptile was plainly not a pachyderm, any more than it had escaped from the ice age. Nor, except occasionally in scientific literature, is it called a monitor. It was, in fact, a Komodo dragon, the largest and, as this Tintin episode usefully demonstrates, probably the most widely misunderstood lizard known to man.

Komodo dragons certainly sound scary. They grow to lengths of between 2 metres and 3 metres (7ft-10ft), weigh up to 68kg (150lbs), and live for up to 50 years. For short bursts, they can run at up to 24kmh (15mph). When young, before they get too big, they climb trees and dive 4.5m underwater. Their scales contain small pieces of bone, making them effectively armour-plated. They are capable of virgin births. They are carnivorous, feasting mainly on carrion but also going for live birds, mammals and, occasionally, people. They have a long forked tongue, big claws and 60 razor-sharp, inch-long teeth. They eat 80% of their body weight at one sitting. And their saliva is deadly poisonous; if you're bitten and get away, you'll probably die of blood poisoning anyway.

Nor has the Komodo's reputation been much enhanced by the events of recent days. "It was big ... It tried to have a go at my feet," was how Helena Nevalainen, a Swedish member of the scuba diving group rescued this weekend from the Indonesian island of Rinca, described to the world's media her chilling encounter with the one she had to fight off three times using her lead-weighted diving belt. "I threw my belt. He came back and bit it, and then he let go. After that he came back one more time. I am just happy I'm here, alive."

But how dangerous are they really? "They're arguably one of the most intelligent reptiles on the planet," insists Ian Stephen, assistant curator of the herpetology department at London Zoo, which has two Komodos. "They demonstrate a real capacity to play, they recognise their keepers, they can distinguish between different people's voices. I'd liken them to a big dog like a doberman, say: they are very powerful and they have the potential to be very dangerous. But like most of these things, the actual risk is wildly exaggerated."

Outside captivity, the Komodo dragon, or Varanus komodoensis, is confined to the islands of Komodo, Rinca, Flores, Gili Motang and Gili Dasami in central Indonesia, which certainly helps as far as reducing risk is concerned. The species was discovered by Europeans only in 1910, when a Lieutenant Steyn van Hensbroek of the Dutch colonial administration heard rumours of a huge and terrifying "land crocodile". These days there are probably fewer than 6,000 of them left in the wild, and they are protected by a raft of national and international laws.

The reptiles have a highly developed sense of smell, and can detect a dead animal from as far as five miles away. Komodos are ambush predators, lying in wait for prey that may include smaller dragons, birds, rats, monkeys, wild pigs, goats, deer and even horses and water buffaloes. Once the victim is close enough the reptile will pounce, going for the soft underbelly or throat of its victim and using its powerful, loosely articulated jaws to tear off big chunks of meat. Smaller prey, up to goat size, is consumed whole; Komodos have been seen charging trees with the dead in their mouths to ram it further down their throats.

"The one danger that's not exaggerated, though, is their saliva," says Stephen. "Any prey that is bitten but escapes will probably die of septicemia within three days." Even a bite from a captive Komodo, whose saliva will contain many fewer than the 70-80 deadly bacteria typically found in the mouth of a wild specimen, will necessitate an intensive course of powerful antibiotics, he warns. (It is in captivity, incidentally, that Komodos have shown themselves capable of what is known as parthenogenesis: in 2005 and 2006, two females at London and Chester zoos, Sungai and Flora, both laid clutches of viable eggs despite having had no contact with male dragons.)

But despite all the scare stories, attacks on humans are very rare. Komodo islanders are used to living around the reptiles: their houses are built on stilts, they keep a close watch on their children, take care when venturing into the undergrowth, and if they do encounter a dragon intent on doing them harm, use sticks and stones to drive it off. In fact, the first recorded fatal attack in 33 years occurred last year, when a dragon attacked an eight-year-old boy who, Stephen says, "wandered into the bushes for a pee and was unlucky enough to come across a dragon in ambush".

Komodos are, however, no respecters of reputations. In another incident, the executive editor of the San Francisco Chronicle, Phil Bronstein - married at the time to Sharon Stone - was badly bitten on the foot by a Komodo at Los Angeles zoo in 2001. Bronstein had to undergo surgery to reattach several severed tendons after a keeper told him to remove his white trainers so the dragon didn't mistake them for the white rats it was usually fed for its meals. ("Whose idea was it," one of the paper's readers later commented, "to remove the white shoes from the white feet of a white man in the hopes of not confusing a near-sighted, simple-minded, ravenously hungry lizard accustomed to a diet of white rats?")

Incredibly powerful, then; potentially dangerous; but on the whole, a bit of a bad rap. Unless you're very unlucky. In which case, best beware of pestilential pachyderms.