"The stories that rise most are those that pose a threat to readers," said Mikkelson, whose website gets more than six million visitors a month. Readers send in questionable emails for Snopes to look into, the couple using a combination of police reports, news clippings, medical journals and encyclopedias to make their findings.

"I might use Google or Wikipedia as a starting point," he told Reader's Digest. "But that's not research."

Well known examples of slacktivism include warnings about carjackers posting flyers on rear windscreens to force drivers out of their cars ("Nothing rules out there having been one car theft carried out in the manner described, but we have yet to hear about it," says Snopes) and the carcinogenic effects of leaving bottled water in hot cars (partly true, is the ruling)

"About a year ago, people started sending us photos from the internet of a freakishly large dog walking alongside two people and a horse, and it made me go, 'Wait a minute,'" Mikkelson said of Hercules, the 105 kilogram English Mastiff "with paws the size of softballs".

"We investigated. The picture turned out to be a digital manipulation, what we call fauxtography."

Fauxtographer s had a field day after the 2004 Boxing Day Tsunami, creating images of giant waves and fantastical deep sea creatures supposedly washed ashore; pictures that have resurfaced and done the rounds again as new "scientific discoveries".

Republican Vice Presidential candidate Sarah Palin was the victim of a particularly bad fauxto, while a picture of First Lady Michelle Obama posing at a soup kitchen for a "homeless" person equipped with a $500 camera phone was also dismissed as a fabrication.

More surprisingly, however, is the number of Snopes investigations that prove true, the bulk of them involving over-sized animals and stupid humans.

While a particularly dramatic fauxtograph of a Great White shark lunging at a British Navy diver dangling from a helicopter was dismissed by Snopes as false, the real equivalent was just as chilling.

A lot of the time, however, a good picture proves just too tempting for urban myth makers, who fill in the gaps with make-believe before pressing 'send'. The Mikkelsons say it is a "common internet phenomenon" and has in the past included celebrity homes, trashy weddings and gruesome medical conditions.

"Someone makes pictures available online, they begin to circulate through email forwards, the original attribution is lost along the way, people begin to make up stories to explain the origins of the now sourceless photographs, and those fabricated explanations become attached to the pictures as they continue to circulate."

Examples include a for-lease Hawaiian holiday property touted as the private home of golfer Tiger Woods, and an elaborate Amsterdam stage set featuring stacked caravans that was circulated as a "redneck mansion".