Just below the surface of the internet sits a network of hidden websites home to some of the worst stuff imaginable. Now hackers are going after pedophiles hiding in the so-called dark net.

Since the beginning of October, a group of hackers associated with the hacktivist collective Anonymous have waged war on an underground forum called Lolita City, where hundreds of users openly trade child porn. They're using hacking tools with nicknames like "Chris Hansen"—after the NBC "To Catch a Predator" host—to bring down the forum and identify users. They call their crusade OpDarkNet.

Lolita City lives on the "dark net," a sort of shadow internet hidden from most internet users. The dark net has sprung up around a service called TOR, which lets anyone host websites and browse the web anonymously with an easy-to-use suite of software. TOR is primarily aimed at protecting activists from oppressive governments, and ensuring privacy for normal internet users.

But it's also become a favorite tool of cybercriminals, hackers, pedophiles and drug dealers. In August, Dutch authorities took down four child porn forums on TOR run by 27-year-old Robert Mikelsons. The underground drug market Silk Road we profiled this summer also uses TOR; the illict activity on the dark net recently attracted the attention of New York senator Chuck Schumer, who demanded the DEA shut down Silk Road.

It also attracted the attention of the team of about a half-dozen vigilante Anonymous hackers now attacking Lolita City. Where Anonymous is often portrayed as dangerous cybercriminals, these hackers wanted to go after criminals themselves.

"We have been targeting them in secret for a while now, taking down their servers as much as possible," one hacker named Arson told us in a chat. "We decided to seek media attention for this operation so that we may get the resources needed to shut them down on a more permanent basis."

As part of their campaign, the hackers infiltrated the members-only board with fake accounts then uploaded scores of clips of "To Catch a Predator," masquerading as child porn. They leaked a list of more than 1,500 Lolita City usernames, and crashed Lolita City's web host—a popular dark net outfit called Freedom Hosting.

"The owners and operators at Freedom Hosting are openly supporting child pornography and enabling pedophiles to view innocent children, fueling their issues and putting children at risk of abduction, molestation, rape and death," the hackers said in a press release. "For this, Freedom Hosting has been declared #OpDarknet Enemy Number One... We will continue to not only crash Freedom Hosting's server, but any other server we find to contain, promote, or support child pornography."

On the dark net, Lolita City is fairly notorious as a den of pedophiles and gets hundreds of thousands of views a day. The hackers claim it hosts more than 100 GB of child porn. In a censored screenshot one of the hackers sent us, Lolita City users are seen discussing an image of what the hacker says is a young girl being abused.

"Would definitely fuck her and cum deep in her cunt mouth and ass," wrote a user named childfugger. There are pages of comments like that—37,079 in all—and this is just one subsection.

On message boards and in chat rooms on the dark net, the hackers' crusade against Lolita City has sparked a firestorm. Many who use the dark net see it as a protected space for free speech and privacy, not for pedophiles. They despise child porn forums because it stains the reputation of the dark net and the TOR network it relies on, and they stage their own crusades against it. For years, computer experts have been trying to build a system to track pedophiles on TOR.

The OpDarkNet hackers say they're motivated by a similar wish to clean out the dark net. Child porn "tarnishes the purpose of TOR… which was originally built to protect people in China and Iran from their government," a hacker named Vicious told me.

But people who frequent the dark net are concerned with the attention the brash hackers are likely to attract. "Nobody likes those guys," a user nicknamed Immanuel said of OpDarkNet. He accused them of "showboating," with their press releases and data dumps. A better way to take down pedophiles would be to quietly forward the information to authorities: "Then the cops could've conducted a real investigation and actually caught people red-handed and in possession of CP [child porn]," he said.

Another user named NoReason said OpDarkNet reflected a recent influx of users that had unsettled the dark net: "A few months ago new people started to arrive to the TOR network sites, I guess it was just a matter of time for something like this to happen."

Indeed, the recent hacker frenzy sparked by hacking groups like Anonymous and LulzSec has raised the profile of the dark net, which had previously only been whispered about on forums and in blog comments. The digital underground is blowing up. And if that new attention flushes pedophiles out of this dark corner of the internet, all the better.