A study has shone some partial light on how widely the Tor network is used to access sites featuring child abuse images and those that allow users to buy illegal drugs.
The results revealed the majority of hidden Tor site requests lead to child abuse pictures, while the second top search was for illegal drugs.
Dr Gareth Owen from the University of Portsmouth, who collated the data over a six month period, said: "It might look like there are lots of people visiting these sites but it is difficult to conclude that from this information. What proportion are people and which are something else? We simply don't know."
Although most of the traffic going through Tor is to legitimate sites that are being used to surf the web anonymously, rather than for illegal activity, 1.5 per cent of all searches were directed at .onion sites.
A total of 80,000 of these sites were uncovered, although the majority were removed after being visited. "Most of the hidden services we only saw once. They do not tend to exist for a very long time," he said.
The most popular content on the .onion sites was associated with illegal drugs, underground markets, fraud sites, mail services and Bitcoin dealerships.
Other services were being used to control botnets and prevent them from infiltrating home networks. However, because many of these botnets now cease to exist, commands were still using Tor to crawl dormant command systems, the BBC explained.
Dr Owen was able to collate the data by setting up servers to join the Tor network, which then tracked services the visitors were using. The system could then follow the users and download HTML files that exposed how many people were visiting the sites and the types of information displayed.
Tor was created to help people stay anonymous when browsing the web, and many of its users are said to be human rights activists who want to protect their identity rather than criminals, Roger Dingledine, one of the Tor network's original developers told the BBC.
