Five years ago this week, the FBI arrested Ross Ulbricht, the alleged mastermind of a massive, illegal online drug market called "Silk Road."

The FBI said the site had nearly a million users and that more than a billion dollars in drugs were sold using bitcoin. It had about 13,000 active drug listings.

Ulbricht was later sentenced to double-life in prison without parole -- but despite the stiff sentence, new sites have risen to take Silk Road's place, and are bigger than ever before.

The largest five dark web markets now have more than 120,000 illegal drug listings, visits to the sites reveal. The listings include just about every drug, ranging from opioids, to weed, to meth.

The markets exist on the "dark web," or “darknet,” where it is extremely hard for authorities to determine anyone’s identity or location. The invention of bitcoin enabled such sites because the currency can be hard to trace to users’ identities.

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That helped Ulbircht’s Silk Road site – the first major online drug market – avoid being shut down for a year and half. The FBI was only able to get him after they noticed that Ulbricht had accidentally used his real name on an internet forum.

The rise in online drug market activity comes even despite a continued law enforcement effort that has shuttered several big sites in the last couple of years. Earlier this year, the FBI also found, arrested, and charged 35 suspected sellers of drugs on such sites.

In January 2018, the Department of Justice created a special team to deal with the problem, called the “Joint Criminal Opioid Darknet Enforcement (J-CODE)” team.

Attorney General Jeff Sessions said that “criminals think that they are safe on the darknet, but they are in for a rude awakening... We have already infiltrated their networks, and we are determined to bring them to justice.”

The European Union’s law enforcement arm, Europol, also created a similar force this year.

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But so far, drug listings on the dark web remain more plentiful than ever.

The largest dark web operation, “Dream Market”, which is not accessible from normal web browsers, has more than 72,000 drug listings by itself.

People buy and sell just about every kind of drug on the site, along with stolen credit card numbers and hacking services. The one drug not for sale is fentanyl, a synthetic opioid so dangerous that even that outlaw market bans it. More than half of opioid deaths in America last year were caused by fentanyl.

The next largest dark web drug site is “Silk Road 3.1”, with more than 30,000 listings. Fentanyl and all drugs appear for sale on that site.

The markets continue to grow because they’re still safer for users than back-alley deals, said Jonathan Birk, CEO of Juliet Bravo Solutions, who trains law enforcement personnel on combatting the dark web.

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“It’s very easy to create an account, list a drug, put it in the mail, and ship it to the customer... there is lower risk than selling it on the street,” he told Fox News. “That is why there are more listings today.”

Birk believes law enforcement’s ongoing efforts are critical to keeping the problem under control.

“Enforcement has had effects on the markets already,” he said. “Many markets stopped selling fentanyl and firearms because of the successful enforcement against those items. New digital currencies, such as Monero, have been created because of successful enforcement tracing bitcoin.”

Monero, a competing currency to bitcoin that is even harder to trace, has gained acceptance on dark websites. The top four dark net sites all accept Monero now, in addition to bitcoin.

But some experts say the task for law enforcement is virtually impossible.

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Perry Metzger of Metzger, Dowdeswell, and Co, an internet security consultancy, said that law enforcement had been mostly unsuccessful in stemming the flow of drugs even offline and that there’s little reason to think they’ll be more successful on the web.

“People have an enormous desire to purchase illegal substances... regardless of whatever law enforcement is put in place,” he told Fox News.

Metzger said that it would be best if the government gave up and ended laws against drugs.

“Having drugs illegal creates far more harm than good. Addicts can’t get proper help, and it adds enormous incentives for people to create violent criminal networks. It seems to have been a failure every time it’s been tried.”

Metzger says that even if authorities could get rid of all bitcoin-like currencies – a nearly impossible task -- criminal networks would still find other ways to move money online, because so much profit is at stake.

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He noted that China spends billions on web censorship, “but people who really want info are still able to get it. If the Chinese can’t prevent that, what hope do authorities have of preventing this?”

Birk, however, is more optimistic.                                 

“Today many local agencies are now involved in investigating dark web crimes. For example, the Portland PD was investigating a deadly drug overdose in Portland that led them to Theodore Khleborod ... they were able to track him to South Carolina and arrest him,” he said. “Through his [dark web] listings they were able to potentially tie him to other overdose deaths.”

Khleborod committed suicide in jail last year.

Birk said authorities should keep up their increasing enforcement against the markets.

“In my opinion, it is not a lost cause, despite the increase in listings,” he told Fox News. “Enforcement has made these markets risky to operate and increased the risk for sellers and buyers.”

The author, Maxim Lott, can be reached on Twitter at @MaximLott