Some specialized search engines discussed here.

^The Invisible Web refers to the part of the WWW that's not indexed by the search engines. Most of us think that that search powerhouses like Google and Bing are like the Great Oracle...they see everything. Unfortunately, they can't because they aren't divine at all; they are just web spiders who index pages by following one hyperlink after the other.^
^But there are some places where a spider cannot enter. Take library databases which need a password for access. Or even pages that belong to private networks of organizations. Dynamically generated web pages in response to a query are often left un-indexed by search engine spiders.^
^...To get a more precise idea of the nature of this 'Dark Continent' involving the invisible and web search engines, read what Wikipedia has to say about the Deep Web. The figures are attention grabbers – the size of the open web is 167 terabytes. The Invisible Web is estimated at 91,000 terabytes. Check this out – the Library of Congress, in 1997, was figured to have close to 3,000 terabytes!^