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The deep web (or invisible web or hidden web) is the name given to pages on the World Wide Web that are not part of the surface web that is indexed by common search engines. It consists of pages which are not linked to by other pages (e.g., dynamic pages which are returned in response to a submitted query). The deep web also includes sites that require registration or otherwise limit access to their pages, prohibiting search engines from browsing them and creating cached copies. Pages that are only accessible through links produced by JavaScript and Flash also often reside in the deep web since most search engines are unable to properly follow these links.

It is estimated that the deep web is several magnitudes larger than the surface web.

How to find “invisible” information sources?
You can give it a try to search a keyword in a search engine and add “database”. This might show up several sources, usually not reveiled by search engines.

• Direct Search - is a growing compilation of links to the search interfaces of resources that contain data not easily or entirely searchable/accessible from general search tools. Although these “general” tools are essential for the retrieval of Internet based data, searchers often fail to realize that a massive amount of information is not easily or entirely searchable/accessible via these search tools. Material “hidden” from the general search tools is said to reside on the Invisible Web.
• CompletePlanet - is a directory of “over 70,000+ searchable databases and specialty search engines.”
• Deep Web Research 2006 - lists invaluable sources for researching the invisible web
• Turbo10 - Digs through the Invisible Web too. Option to add a great number of data sources to “collection”.
• Academic Info -
• Yahoo Subscription Search - Yahoo! Search Subscriptions enables you to search access-restricted content such as news and reference sites that are normally not accessible to search engines.
  Note: You will need a valid subscription to the sources above to access their content.
• Public Records Directory - finds hidden sources throughout US and many countries worldwide
• IncyWincy -
• MedBioWorld -