Why Go Directly to Specific Websites?
Many of the pages that you will find using the search engines on
the websites listed here are NOT indexed by general-purpose search
engines (Altavista, Google, etc.) and are therefore invisible to them: such pages belong
to what is known as the "invisible web", and the only way to
access them is by going directly to the website in question and using the website's
own search engine.
The invisible web is said to be around 500 times larger than the portion of the web
currently accessible through general-purpose search engines.