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The Web has been rapidly "deepened" by the prevalence of databases online. With the potentially unlimited information hidden behind their query interfaces, this "deep Web" of searchable databses is clearly an important frontier for data access. This paper surveys this relatively unexplored frontier, measuring characteristics pertinent to both exploring and integrating structured Web sources. On one hand, our "macro" study surveys the deep Web at large, in April 2004, adopting the random IP-sampling approach, with one million samples. (How large is the deep Web? How is it covered by current directory services?) On the other hand, our "micro" study surveys source-specific characteristics over 441 sources in eight representative domains, in December 2002. (How "hidden" are deep-Web sources? How do search engines cover their data? How complex and expressive are query forms?) We report our observations and publish the resulting datasets to the research community. We conclude with several implications (of our own) which, while necessarily subjective, might help shape research directions and solutions.