Websites which offer illegal drugs over the so-called dark net have more than doubled in the last year as reported by BBC News:
One of the most popular access methods for the dark net is the TorBrowser. It allows people to use Tor, an "onion-routing" system that makes a PC's net address untraceable. It bounces encrypted data through several randomly selected computer servers on a volunteer network before it reaches its destination. There are also many hidden sites on the network ending in the dot-onion suffix, including drugs markets.
Last October the general public became aware of the dark net when the FBI took down the online forum Silk Road which allegedly was used by some as an underworld bazaar to peddle drugs and other illicit products and services in virtual anonymity, and arrested its founder Ross William Ulbricht who is accused of contracting for the murder of those who posed a threat to the site as reported by Andy Greenberg for Forbes.
The dark net also is used for the distribution of child porn by organized crime. For example, last June Italian authorities announced it had closed several child porn sites on the dark net which were operated by the 'Ndrangheta or Calabrian Mafia as reported by ANSA.
Of course, these sites aren't so hidden that law enforcement can't find them, and no doubt the intelligence agencies are developing software which can monitor the dark net and trace its users.