http://www.weblens.org/invisible.html

Searching the invisible web (deep web, hidden web): WebLens search portal

Searching the Invisible Web (Hidden Web, Deep Web)

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The terms invisible web, hidden web, and deep web all refer to the same thing: a massive storehouse of online data that the search engines don’t capture. That’s because terabytes of information are buried in databases and other research resources. Searchable and accessible online but often ignored by conventional search engines, these resources exist by the thousands. Known in research circles as the invisible, deep, or hidden Web, this buried content is an estimated 500 times larger than the surface Web, which is estimated at over four billion pages. This mass of information represents a potent research resource, no matter what your discipline or interest. Below are some tools to help you mine it. Students and serious researchers may also find our collections of web directories and scholarly and academic research tools and strategies useful.