Fifteen months after his arrest, Ross Ulbricht, the man accused of setting up and running the black market website known as Silk Road, has gone on trial. Other similar sites, using the so-called hidden or "deep" part of the internet to engage in criminal activity, have also been closed down. But has the crackdown had any effect? And how difficult is it now to access this mysterious part of the internet?

It's hidden from normal internet browsing software like Internet Explorer or Chrome by a complex series of connections, each designed to make your identity more difficult to trace. But the so-called darknet remains simple enough to access.

By downloading and installing a free web browser called TOR, anybody can connect to the internet anonymously. Then, with the help of a few site lists which are available through Google, you have immediate access to a wide variety of goods and services. Among them are a host of illegal offerings - drugs, weapons, stolen credit cards, counterfeit money, hackers and hit-men for hire.

'Neither invisible nor untouchable'

"You don't know who you're dealing with on the markets," said Anupam Chander, a professor of law at the University of California, Davis and author of the book The Electronic Silk Road. "Anonymity is built into these markets, you don't know whether the person on the other side is genuine or actually a policeman waiting to trap you."

One of the pioneers of underground online marketplaces was the site Silk Road. When law enforcement agencies arrested the man they accuse of setting it up in October 2013, the FBI claimed it had turned over $1.2 billion in just over two and half years, netting its owners more than$80 million in profits.

"Law enforcement tried to reduce the confidence of these markets by arresting folks like the alleged operator of Silk Road or any of the iterations of it," said Lillian Ablon, a cyber-security researcher at RAND Corporation. "But essentially these markets just bounce back - there is high confidence and low risk."

In November 2014 an international police operation took down at least 27 darknet websites, resulting in 17 arrests.

"For a long time criminals have considered themselves beyond reach," Troels Oerting, head of Europol’s European Cybercrime Centre said at the time. "We can now show that they are neither invisible nor untouchable. The criminals can run but they can’t hide."

Darknet bounces back

But since the operation numerous new sites have appeared. Many of them are now believed to be run outside the US and despite the crackdown, researchers are reporting renewed activity.

"The successor markets are presumably run out of Eastern Europe," said Nicholas Weaver, a researcher in networking and security at the International Computer Science Institute. "A long-term way these markets are disrupted is not by attacking the markets themselves, because the market can move overseas, but law enforcement needs to focus on the local dealers and their financial infrastructure."

Publicity around the arrests and shutdowns also appeared to have increased public interest, in particular in the illegal goods and services on offer.

"Participants who are in the black markets learn from what law enforcement is doing and change their tactics, adapt their tactics," said Ablon. "People who are not in the markets are now aware of the low-risk, high-gain of getting into these markets or the products that they could buy.”

The online markets also appeared to be moving their activity to more obscure parts of the internet. One report suggests a new version of the Silk Road website is now running on a network called I-2-P, an even deeper, darker part of the network. It's reported to be much harder to access - a new challenge law enforcement agencies are no-doubt already tackling.

"Illicit markets will continue just as they have existed forever," said Chander. "New thieves will always come up, new people will be trying to take advantage of the money to be made."