The invisible web is what you can’t find using the usual search engines or web directories. The search engines have adapted so they can find some of the invisible web. Pages not created with standard HTML, sites created with other software or scripts didn’t always show up in searches.
So what’s still unavailable? Databases which use passwords or choose to block or restrict search access in some way. Anything which requires user interaction in order to use the site, becomes unavailable or invisible to standard searches.
You probably think this hidden content doesn’t really matter. You’ve found the information you need online and don’t see anything lacking. But, not everyone uses the web the same way. Those who want to research a topic or find detailed information may want a peek at the resources in the deep web. There is a report stating the invisible/ deep web is 500 times bigger than the standard web. Doesn’t that make you at least a bit curious?
You can investigate some of these extra resources yourself, even using a standard surface search. When you type in your search terms add words like: database, portal, directory to it. You may land at some page in a larger site and need to do some backtracking or know a bit about mining a site for it’s resources. However, it could be interesting to explore what’s out there in the deep, dark, invisible land of the WWW.
About.com: The Invisible Web
Internet Tutorials: The Deep Web
JEP: White Paper: The Deep Web: Surfacing Hidden Value
Wikipedia: Invisible Web
Wikipedia: Dark Internet
Those Dark Hiding Places: The Invisible Web Revealed (No longer being kept updated).
Search Engine Watch
Resources for Searching the Deep Web: