a.k.a. dark Web, deep Web, area 51 of the Internet
The invisible Web refers to the vast repository of information that search engines and directories don't have direct access to, such as databases. It is true that information from the invisible Web is "buried so deep" that it doesn't show up in most organic search results, and it is estimated that the invisible Web is 500 times larger than the visible Web (or surface Web).
Basically the reason why such a large portion of the Web is not indexed by search engines is because it consists of dynamic pages within database-drivenWeb sites. Deep Web URLs are identifiable in that they are very long and contain symbols, such as question marks or percentage signs. If you are doing an onlinesearch and can't find what you're looking for, try using a Web site's site search to delve deeper.