The Invisible Web

The Invisible Web consists of "[t]ext pages, files, or other often high quality information available via the World Wide Web that general purpose search engines cannot, due to technical limitations, or will not, due to deliberate choice, add to their indices of Web pages. Sometimes also referred to as the "Deep Web" or "dark matter."
(Chris Sherman and Gary Price, The Invisible Web: Uncovering Information Sources Search Engines Can't See, 2001, p. 57.)

Information found in the Invisible Web may include files with these formats:

• PDF or Postscript (except Google)
• Flash
• Executable programs
• Compressed files (.zip, .tar, etc.)
  (Sherman and Price, p. 58)

Another aspect of the Invisible Web is the "private" Web contained in most academic libraries. These include the electronic and scholarly resources such as databases, full-text journal collections, image collections, and more.

Many of the resources on the Invisible Web are subject-related. An excellent place to begin is The Invisible Web Directory (http://www.invisible-web.net/). Select your area of interest and begin searching!