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If you looked up stingrays on Wikipedia last week, you would have learned that, as well as living in tropical coastal waters and reproducing in litters of five to 10 offspring, the cartilaginous marine fish also "hate Australian people".

It wouldn't take long to realise that the last bit isn't true, or certainly that no one asked stingrays whether it was.

In fact, it was a piece of "internet vandalism" by a growing band of cyberspace guerrillas who are targeting sites such as Wikipedia.

Since the death of Steve Irwin - the Australian television naturalist who was struck in the chest by a stingray's barb last year - the entry has become one of the online encyclopaedia's most regularly vandalised articles.

The net has always been a magnet for bored people looking for amusement, but while some write a blog and others search for pornography, a growing number sabotage high-profile websites.

Any site that relies on well-meaning contributions from the public is vulnerable - MySpace, Slashdot and YouTube have all been victims - but Wikipedia is susceptible because anyone can edit it, including those whose sole aim is to undermine it.

Wikipedia relies on the goodwill of loyal community members to weed out malicious contributions.

Launched in 2001, it quickly became the world's most popular non-profit website and is among the top 10 visited on the net, containing more than six million articles in 250 languages. With only seven paid administrative employees, the encyclopaedia depends on the altruistic collaboration of its four million registered users, as well as countless others who edit the site anonymously.

It has become very powerful in the world of the internet and is used as a reference tool for everyone from students to politicians.

Wikipedia has been accused of being unreliable and inconsistent, and in 2005 American journalist John Seigenthaler blasted it as "flawed and irresponsible" after libellous claims were introduced into his biography.

Last month it was revealed that a prominent editor, Essjay, had lied about his identity to win arguments with other Wikipedians. Essjay claimed to be a tenured university professor on his personal user page, when he was a 24-year-old community college drop-out.

Wikipedia faces a controversy that could undermine its whole philosophy - a community of people free to add and improve on its entries. Vandals want to add, but improve is not in their vocabulary.

There are some obvious targets for the Wiki vandals, such as the articles on George W. Bush, Britney Spears, abortion and Islam, as well as more bizarre choices, such as the pages on Bigfoot, elephants, bananas and liver.

Articles covering current events, such as the death of Anna Nicole Smith, are often sabotaged.

"The more popular Wikipedia becomes, the more vandals it attracts," says a spokesman from the Wikimedia Foundation, which runs Wikipedia's administration.

"But the older we get, the more we are developing ways to fight it. We have thousands and thousands of volunteers worldwide who are dedicated to the project and work on combating vandalism every day."

Theresa Knott is one such devoted Wikipedian. She visits the site daily, often editing at 5.30am before she leaves for work as a London primary school teacher. Her efforts have been rewarded with regular abuse from vandals and kudos from her Wikipedia peers, who elected her to the position of administrator in 2003.

This means she is one of more than 1000 Wikipedians who have special powers to lock down vulnerable articles from further editing, and temporarily block problem users from making changes to the site.

"I'm sure that most of the people who do it are young," she says. "They write the same kind of things over again: it's always 'My friend is gay', 'My teacher is gay'. It's part of the general anti-authority thing kids have."

The more damaging edits come from problem editors who register a user name in the hope of gaining online notoriety. Alm93 caused havoc last year by making subtle and inaccurate changes to important statistics, such as population figures and average human height.

The standard way to stop a vandal is to "revert" their changes and leave a warning on their personal user page.

Most then lose interest, but if they persist Knott will block them for a limited period, say 31 hours, or longer if they are a repeat offender.

Wikipedia is remarkably robust in the face of attacks. Most casual users are unaware of the problem because vandalism tends to be spotted quickly: the comments on the stingray article lasted less than five minutes.

The more insidious edits are spotted by an army of editors who search for sabotage.