Dark web: What lurks beneath the surface of the internet?

The alleged abduction of a British model in Italy has brought renewed attention to the dark web, after she claimed she was almost "sold" online.

Chloe Ayling, 20, says she was drugged, kidnapped and held captive for a week near Turin before being released at the British Consulate in Milan, Italian newspaper Il Giornale reports.

"Her kidnapper allegedly threatened to auction her on the internet while trying to extort €300,000 (£270,000) from her agent," Sky News reports.

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Lukasz Pawel Herba, a Polish national who lives in the UK, has been arrested on suspicion of kidnapping for extortion purposes. Italian police are still searching for other suspects.

Herba says he was working for the Black Death Group, which claims to sell drugs, weapons and sex slaves on the dark web.

But what exactly is the dark web?

What is the dark web?

The dark or hidden web refers to a part of the internet that cannot be found using Google or other regular search engines and is only accessible with a special software product, one of the most popular of which is Tor (The Onion Router) Project.

Tor makes a computer's net address invisible, providing the closest thing to anonymity on the internet. It encrypts data in multiple layers, like an onion, and then sends that information through multiple relays, each one of which peels off a layer before the data reaches its destination.

What are the dangers of the dark net?

The National Crime Agency (NCA) says there has been a rise in the use of the dark web as a marketplace for firearms, drugs and indecent images of children.

The notorious Silk Road website, which was taken offline in 2013 and its alleged owner arrested by the FBI, allowed visitors to buy drugs, fake IDs and firearms, as well as trade tips on how to hack computers and ATMs and even hire assassins.

A BBC investigation in 2014 also found the dark web was used by tens of thousands of paedophiles to anonymously trade images of sexual abuse. One site received as many as 500 page views per second, while data from another suggested Britons are heavily involved in producing and distributing obscene images of children.

It is always used for criminal activity?

No. Tor was initially funded by the US government and is often used by pro-democracy campaigners, whistleblowers and journalists operating under repressive regimes. For example, it was used by activists during the Arab Spring to avoid detection and by Chinese citizens to get around the country's "Great Firewall".

The people behind the Tor Project insist it protects innocent users from crimes such as identity theft and stalking. In particular, it has been hailed as a safe place for victims of domestic abuse to seek support without detection from their attackers.

Co-creator Roger Dingledine criticised journalists for sensationalising the size and scale of the dark web, the BBC reports.

"I would say that there are bad people on the internet and they're doing bad things, but Tor does not enable them to do the bad things," he said. "It's not like there's a new set of bad people in the world who exist because Tor exists."

Can it be cracked?

Governments around the world recognise the threat from the dark web, but not one appears to have found a steadfast way to get around the system. Security specialists believe there are innovative ways to unmask the users of paedophile sites, such as using complex algorithms to mine chat rooms for data. However, this relies on the user revealing a certain amount of information.

Whistleblower Edward Snowden – who used encryption software to leak classified documents to The Guardian and Washington Post – revealed the US National Security Agency's frustrations with the dark web. In one top-secret presentation, entitled Tor Stinks, the NSA said: "We will never be able to de-anonymise all Tor users all the time" but "with manual analysis we can de-anonymise a very small fraction".

The people behind Tor says its key role in facilitating free speech, privacy and protection for victims outweighs the argument to build a "backdoor" into the system for law enforcement agencies.

"There have been rumours that law enforcement has cracked Tor but, aside from isolated vulnerabilities, Dingledine says the concept remains solid," the BBC reports.