The Deep Web (also called the Invisible Web, Undernet, and the Hidden Web)
Search engines rely on technology that generally identifies "static" pages, rather than the "dynamic" information stored in databases. Deep Web content resides in searchable databases, the results from which can only be discovered by a direct query. Without the direct query, the database does not publish the result. Thus, while the content is there, it is skipped over by traditional search engines which cannot probe beneath the surface.
"Information on the deep Web is 400 to 550 times larger than the commonly defined World Wide Web."
The furthest corners of the Deep Web, known as the Dark Web, contain content that has been intentionally concealed.
- Bright Planet, "Deep Web: Advanced" (full-text).
- Bright Planet, "Deep Web: A Primer" (full-text).
- InfoSec Institute, "Diving in the Deep Web" (Mar. 14, 2013) (full-text).