The Visible Web is just the tip of the iceberg
The invisible web (also called deep web) is that part of the Web that cannot be crawled or indexed by traditional search engines. These pages may often link to searchable databases on web sites that can only be generated if queried from within the site itself. Search engines typically do not search these databases, but you can search them manually if you know where to look.
The invisible web consists of content from thousands of specialized, searchable databases made available via the web. Some invisible Web sources include:
Manufacturer websites often consist of product catalogs with detailed spec sheets, product images and item descriptions.
The US patents and Trademarks office that contains all the patents and trademarks registered in the country.
Online health database consisting of diseases & conditions, news, medication and other health information.
Travel websites with travel fares and availability. Yahoo! Farechase is a well-known example.
Invisible web search technology can be used to build “Vertical Searches” or “specialized searches” that focus on specific topics or business use.
Invisible or Deep Web: What it is, Why it exists, How to find it, and its inherent ambiguity
About the Deep Web
Searching the Invisible Web
The Deep Web