10 Search Engines to Explore the Deep End of the Invisible Web
http://www.makeuseof.com/tag/10-search-engines-explore-deep-invisible-web/

he 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.

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