Research Beyond Google: 119 Authoritative, Invisible, and Comprehensive Resources
http://oedb.org/library/college-basics/research-beyond-google

OEDb
2006

Abstract:

Google, the largest search database on the planet, currently has around eight billion web pages indexed. That's a lot of information. But it's nothing compared to what else is out there. Google can only index the visible web, or searchable web. But the invisible web, or deep web, is estimated to be 500 times bigger than the searchable web. The invisible web comprises databases and results of specialty search engines that the popular search engines simply are not able to index.