the invisible web
Although "general" search engines are essential for the retrieval of Internet based data, searchers often fail to realize that a massive amount of information is not easily or entirely searchable/accessible via these search tools. Material "hidden" from the general search tools is said to reside on the Invisible Web. (Gary Price, MLIS).
Use these tools to uncover information sources that search engines can't see.
ERIC - Education Information
PubMed - National Library of Medicine