The Deep web reference article from the English Wikipedia on 24-Apr-2004
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Deep web

Helping orphans the way you would do it
The deep web or invisible web is the large part of the publicly accessible pages on the World Wide Web that is not indexed by search engines. It consists mainly of dynamically generated pages that are founded on databases and are responses to queries.

Search engine use web crawlers that follows 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.

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