The invisible web is defined as: the portion of the web not reachable or searchable via standard search engines. Contents of specialized databases, whether free or fee-based, are not always located using search tools such as directories or search engines. The valuable information within these databases has been called the invisible web. Database contents are searchable only by entering the individual database or repository. Examples of some of the largest invisible or "deep" web sites not fully accessible by general search engines are
Based on your use of the above sites you recognize that best and most comprehensive searching and retrieval is done from the site itself, using the full functionality of the native interface to access contents.