SAN FRANCISCO - No sooner had authorities announced the shuttering of an alleged illegal online drug bazaar than another popped up claiming to take its place.

Welcome to the "dark Web," an increasingly popular corner of the Internet where thousands of computer users from around the globe interact anonymously - and, in many cases, illegally.

On Thursday, the U.S. Department of Justice charged a 26-year-old San Francisco man with operating Silk Road 2.0, an anonymous website which authorities say rang up $8 million in monthly drug sales.

On Friday, an underground website calling itself Silk Road 3.0 Reloaded claimed to be open for business on the TOR network, which is linked globally through special browsers that encrypt Internet traffic. Several other websites on the TOR network also claimed to be open for drug transactions.

The dark Web poses new and formidable challenges for law enforcement agencies around the world that have been dealing for decades with more conventional international drug trafficking. The reach and anonymity of these 21st century Internet operations is difficult to penetrate. Silk Road and copycats on the TOR network are not readily visible through popular Internet search sites. The buyers and sellers don't exchange cash, dealing instead in often untraceable digital currencies, usually Bitcoin. So there are no banking records for investigators to subpoena.

"As long as the dark Web exists, there will always be people who set up places to engage in wrongdoing," said Joseph DeMarco, a defense attorney and former federal prosecutor who headed the computer crimes section of the U.S. attorney's office in New York. DeMarco said he was skeptical that a single "global solution" would be found to stop illegal activity on the TOR network.

"There will always be an arms race between the bad guys and law enforcement," DeMarco said.

Those who created and support the TOR network say it's a way to protect online users' privacy in the digital age. TOR boasts that none of its websites will appear in Google search.

"TOR was created to protect people's privacy and anonymity, and we don't condone its use for these illegal activities," said Roger Dingledine, who co-created the TOR network originally for the U.S. Navy.

But investigators around the globe say the network is also a place of flagrant and profligate illegal activity of all sorts - from prostitution to arms trafficking - and they vow to crack down.

"Underground websites such as Silk Road and Silk Road 2.0 are like the Wild West of the Internet, where criminals can anonymously buy and sell all things illegal," said Homeland Security Investigations Executive Associate Director Peter Edge.

The day after the FBI announced it had arrested Blake Benthall in San Francisco, European authorities said they arrested 16 other people in Ireland and Germany as part of the crackdown on dark Web sites.

"They believed that they could not be touched. We've proven that is not right," said Troels Oerting, head of the European police agency's cybercrimes division. "Expect to see more of these operations in the future."

Oerting expects more than 55 websites will be shut down.

In addition to the Silk Road site, authorities say they have seized or shut down other virtual marketplaces with names such as Hydra, Cloud Nine, Pandora and Blue Sky. Police in Europe seized $1 million in digital currency and $225,000 worth of cash, drugs, gold and silver.

The FBI said it seized $100,000 in cash from Benthall's San Francisco apartment and allege that he earned $400,000 in monthly commissions on $8 million in monthly sales.

Benthall, who has worked as a computer software engineer, was in federal custody in San Francisco awaiting U.S. marshal transportation to New York to face felony charges of conspiracy to traffic in controlled substances, computer hacking, conspiracy to traffic in fraudulent identifications and money laundering. Benthall has not entered a plea.