One day last summer, Google’s search engine trundled quietly past a milestone. It added the one trillionth address to the list of Web pages it knows about. But as impossibly big as that number may seem, it represents only a fraction of the entire Web.

Beyond those trillion pages lies an even vaster Web of hidden data: financial information, shopping catalogs, flight schedules, medical research and all kinds of other material stored in databases that remain largely invisible to search engines.

Search engines rely on programs known as crawlers (or spiders) that gather information by following the trails of hyperlinks that tie the Web together. While that approach works well for the pages that make up the surface Web, these programs have a harder time penetrating databases that are set up to respond to typed queries.

“The crawlable Web is the tip of the iceberg,” says Anand Rajaraman, co-founder of Kosmix (www.kosmix.com), a Deep Web search start-up whose investors include Jeffrey P. Bezos, chief executive of Amazon.com. Kosmix has developed software that matches searches with the databases most likely to yield relevant information, then returns an overview of the topic drawn from multiple sources.

This level of data integration could eventually point the way toward something like the Semantic Web, the much-promoted — but so far unrealized — vision of a Web of interconnected data. Deep Web technologies hold the promise of achieving similar benefits at a much lower cost, by automating the process of analyzing database structures and cross-referencing the results.

“The huge thing is the ability to connect disparate data sources,” said Mike Bergman, a computer scientist and consultant who is credited with coining the term Deep Web. Mr. Bergman said the long-term impact of Deep Web search had more to do with transforming business than with satisfying the whims of Web surfers.