With the rise of commerce on the Web, and the recognition of the market value that certain types of information can command, parts of the Web have been locked away, hidden behind corporate intranets and firewalls, passworded sites, and subscription services.

A high proportion of this Deep or Invisible Web is hidden from view, whether from you and I, or from search engines. Estimates of the size of this deep Web vary, but most agree that it is many times the size of the Surface Web, i.e. the web that is open and accessible to the public.