The term Deep Web (also called the Invisible Web and the Dark Web) refers to the hidden web content not indexed by standard search engines. Some estimates are that the Deep Web is 500 times larger than the surface Web (the visible Web). Think of the surface web as the surface of the ocean-miles and miles of surface out there, as far as the eye can see. But when you cast a net, it goes below the surface and captures things unseen to the eye. dark web

Why is the Deep Web invisible? Because its hard-to-find web sites and search engines:

May have inadequate links to their content

Require users to register

Have spotty indexes to their content.
For more information on the Deep Web, check out the following sites:

deepwebresearch.info: monitors Invisible Web research resources and sites on the Internet

brightplanet.com: collects known, unknown, and hidden content from formerly inaccessible web sources

completeplanet.com: a directory of over 70,000 searchable databases, organized by content and subject categories.
The following are examples of Invisible Web people search databases:

411x411.com: Directory assistance and people search databases.

123people.com: Comprehensive search engine that also pulls from Deep Web sources as well. It also offers international searches.

pipl.com: Another comprehensive search engine that pulls from Deep Web sources. You can search by phone number, email address, even business names.

cvgadget.com: This has a simple interface-just plug in a name. The results are categorized by various Google search engine utilities (news, images, documents, etc.). Other categories are listed by various social networking sites, blogs, business networking sites, and so forth.
How can you dive into the Deep Web? Simple. Add the words "search" or "database" (without the quotes) to your queries to bring those hidden databases and directories to the surface.