The Deep Web, also known as the Invisible Web, is a portion of the web not reached by standard search engines, such as Google and Bing. Less than 10% of the web is indexed by search engines, with the remaining 90% of web content called the Deep Web. It is estimated to be 2-500x bigger than the visible web.  Much grey literature is located in the Deep Web, and many of the tools on this page will help to locate it there.  In addition, there are some search engines and databases designed specifically for the Deep Web.