The Deep Web (also called the Deepnet, Dark web, DarkNet, the Invisible Web, the Undernet or the Hidden Web) is World Wide Web content that is not part of the Surface Web, which is indexed by standard search engines. It should not be confused with the dark Internet, the computers that can no longer be reached via the Internet, or with a Darknet distributed filesharing network, which could be classified as a smaller part of the Deep Web. There is concern that the deep web can be used for serious criminal activity.
Estimates based on extrapolations from a study done at University of California, Berkeley in 2001, speculate that the deep Web consists of about 7.5 petabytes. More accurate estimates are available for the number of resources in the deep Web: research of He et al. detected around 300,000 deep web sites in the entire Web in 2004, and, according to Shestakov, around 14,000 deep web sites existed in the Russian part of the Web in 2006.