Chris Shearman's site Searchwise is fairly well known and he made the invisible-web search engine that is now under reconstruction. Another IncyWincy: The Invisible Web Search Engine is still running.
See also: 'The Invisible Web: Uncovering Information Sources Search Engines Can't see ', Ariadne Issue 30
Seemingly everything can be found on Google and this http://www.cantfindongoogle.com/ site is now broken.
But this is not about the invisible web that SE's don't index and we know how Google find the needle in the haystack.
It is about the Dep Web or information that is in databases. Is the following surprising news to you?
"The Web is indeed huge! In fact, it's so big that it's hard to get an accurate count of the size. By January 2004, it was estimated that the Web contained over 10 billion pages, with an average size of 500 Kb. With a world population of about 6.4 billion, that's almost 2 pages for each inhabitant. The early exponential growth of the Web has slowed recently, but it is still the largest document collection in existence. The Berkeley information project "How Much Information," estimates that the amount of information on the Web is about 20 times the size of the entire Library of Congress print collection. Bigger still, a company called BrightPlanet sells access to the so-called Deep Web, which they estimate to contain over 92,000TB of data spread over 550 billion pages. BrightPlanet defines the Deep Web as the hundreds of thousands of publicly accessible databases that create a collection over 500 times larger than the Surface Web. Deep webpages can not be found by causal, routine surfing. Surfers must request information from a particular database, at which point, the relevant pages are served to the user dynamically within a matter of seconds. As a result, search engines cannot easily find these dynmaic pages since they do not exist before or after the query. However, Yahoo appears to be the first search engine aiming to index parts of the Deep Web."
Amy N. Langville and Carl D Meyer: (2006) Google's PageRank and Beyond: The Science of Search Engine Rankings.