Def.: everything a search engine does not see is invisible

1. non linked web servers and pages

2. "dangerous" pages/URLs (?URLs: http://...?... - crawler traps)

3. database content (accessible by textboxes etc. only)

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1. a) servers: detectable by domain registry

   b) pages: important amount detectable by path-crawling

2. "dangerous"/dynamic pages: robust crawler software

3. databases (textbox only etc.):

   a) theoretically by automated (dictionary) input

   b) practically by meta searching the databases

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How big is all that invisible web??