They are known as the "dark Web" — encrypted corners of the internet that promise anonymity to customers who want to buy or sell illegal drugs, weapons and other contraband.
But these futuristic marketplaces recently became much less anonymous after an international sting captured the addresses of thousands of users and shut down two of the biggest sites: first AlphaBay in early July, and then Hansa Market at the end of the month.
Now, many users are wary of joining the next secretive marketplace, and that's exactly the point.
"Don't be stupid and hop on the next big market," one user wrote on the Reddit discussion forum where users openly trade tips on dark Web markets. "It will most likely be completely run by [law enforcement]."
U.S. and European law enforcement authorities say the closures of AlphaBay and Hansa Market were the largest dark Web criminal marketplace takedown in history.
To dark Web users, the message is clear, said Europol Director Robert Wainwright: "You're not as safe, as anonymous, as you think you are."
For U.S. law enforcement agencies, dismantling the criminal dark web has taken on added urgency in recent years amid a deadly opioid epidemic that feeds off these sites. According to a 2015 study, drug forums and contraband markets made up the largest category of sites on the TOR network.
AlphaBay and Hansa were two of the top three criminal markets on the dark Web, sites that sprang up in the wake of drug market Silk Road's takedown in 2013.
Hansa's users numbered in the five digits; AlphaBay had more than 200,000 customers and 40,000 vendors, making it 10 times as large as Silk Road. It generated nearly $1 billion in sales.
The operation to shutter AlphaBay and Hansa grew out of several independent investigations, according to U.S. Deputy Attorney General Rod Rosenstein.
The investigation into AlphaBay appears to have started as early as 2015 when undercover agents posing as customers started making small purchases on the site. In one case, an agent bought an ATM skimming device; in another, an undercover officer purchased a small quantity of drugs.
In December 2016, investigators got a break when they came across a priceless clue: the site operator's personal email address. In the days after AlphaBay's launch in December 2014, investigators learned, the administrator included his personal email address — [email protected] — in AlphaBay's "welcome email" to new users singing up for the site's discussion forum.
It was the kind of gaffe that had exposed Silk Road's founder and would lead to the downfall of AlphaBay's creator.
The email address was traced to Alexandre Cazes, a French-speaking Canadian website designer from Quebec. Born in 1991, Cazes had posted the email address on a tech forum as far back as 2008 and later used it to create PayPal and LinkedIn accounts.
Meanwhile, Europol provided Dutch law enforcement authorities with a lead on Hansa Market that would allow them to identify the site's administrators and locate its servers in Lithuania, Germany and the Netherlands.
"When we knew the FBI was working on AlphaBay, we thought, 'What's better than if they come to us?' " Petra Haandrikman, leader of the Dutch investigative team that brought down Hansa, told cybersecurity blogger Brian Krebs.
Investigators then coordinated the timing of the two sites' takedown. A plan was hatched: The Dutch would move in first, followed by the Americans.
On June 20, as German police arrested Hansa's two German administrators in Germany, Dutch law enforcement authorities moved to seize control of the site. The takeover was seamless.
On July 4, the FBI took AlphaBay offline but made it look like an outage. Unaware that the FBI was on his tail, Cazes swung into action to bring the site back online.
When Thai police, assisted by FBI and U.S. Drug Enforcement Administration agents, raided Cazes' house in Bangkok the next day, they found he'd contacted AlphaBay's server host to request a reboot and was logged into its forum to answer comments by AlphaBay users.
On his unlocked, unencrypted laptop, agents found passwords for AlphaBay, its servers and other online identities associated with the site.
As rumors swirled that AlphaBay operators had absconded in what is known as an "exit scam," authorities sought to quell the talk: AlphaBay was down for maintenance and would be up again soon, they posted on Reddit on July 6.
In the days that followed, the number of users on Hansa jumped 800 percent as AlphaBay users streamed in, according to Wainwright of Europol. To cope with the flood of orders, authorities temporarily closed registration to new users.