"The Invisible Web is composed of web documents which are badly indexed by the broad-based conventional search engines" (or not indexed at all) "Well known databases such as LexisNexis, Dialog… Factiva - only constitute 1% of the Deep Web!“ -- Digimind, 2007
There are many ways to search the deep web; here are but a few:
Nobody really knows! It is always changing.
“We found that the deep Web measured 450,000 Web *databases, among which 348,000 were structured.” Only a very small percentage of these are searchable from surface web search engines like Google.
* Databases are structured, searchable web sites containing distinct collections of objects in text, image, audio, video…format.
source: Structured Databases on the Web: Observations and Implications (highly cited paper from 2004)