The Deep Web resources and site descriptions featured on this page were adapted from the book Going Beyond Google AGAIN, by Jane Devine & Francine Egger-Sider.
What is the Deep Web?
No matter how good you are at using web search engines (Google, Bing, Yahoo, etc.), there are valuable resources on the web that commercial search engines will not find for you. You can get to most of them if you know the URL, but they will not show up in the results list of a search using the most popular "go to" search engines.
These resources, often referred to as the "deep web," the "hidden web," or the "invisible web" contain a variety of content, including databases of articles, data, statistics, and government documents. The term invisible here refers to "invisible to search engines." There is nothing mysterious or mystical involved.
~From The Extreme Searcher's Internet Handbook: A Guide for the Serious Searcher