Deep web
The deep web or invisible web is the name given to the publicly accessible pages on the World Wide Web that are not indexed by search engines. It consists mainly of dynamically generated pages that are based on responses to database queries.
Search engines use web crawlers that follow hyperlinks. Such crawlers typically do not submit queries to databases due to the potential infinitude of queries that can be made to a single database.
In a 2000 study by the search company BrightPlanet, the inaccessible part of the web was estimated to be about 500 times larger, in terms of number of documents, than what search engines already provide access to. Any such figures must be taken with caution, however, due to the difficulty of distinguishing between genuinely different documents and documents that merely represent different database views of the same content.
References
- Gary Price & Chris Sherman. The Invisible Web : Uncovering Information Sources Search Engines Can't See. CyberAge Books, July 2001. ISBN 091096551X
