The deep web, invisible web, or hidden web are parts of the World Wide Web whose contents are not indexed by standard web search-engine programs. This is in contrast to the "surface web", which is accessible to anyone using the Internet. Computer scientist Michael K. Bergman is credited with inventing the term in 2001 as a search-indexing term. Deep web sites can be accessed by a direct URL or IP address, but may require entering a password or other security information to access actual content. Uses of deep web sites include web mail, online banking, cloud storage, restricted-access social-media pages and profiles, and web forums that require registration for viewing content. It also includes paywalled services such as video on demand and some online magazines and newspapers.

Terminology

The first conflation of the terms "deep web" and "dark web" happened during 2009 when deep web search terminology was discussed together with illegal activities occurring on the Freenet and darknet. Those criminal activities include the commerce of personal passwords, false identity documents, drugs, firearms, and child pornography. Since then, after their use in the media's reporting on the black-market website Silk Road, media outlets have generally used 'deep web' synonymously with the dark web or darknet, a comparison some reject as inaccurate and consequently has become an ongoing source of confusion. Wired reporters Kim Zetter and Andy Greenberg recommend the terms be used in distinct fashions. While the deep web is a reference to any site that cannot be accessed by a traditional search engine, the dark web is a portion of the deep web that has been hidden intentionally and is inaccessible by standard browsers and methods.

Non-indexed content

Bergman, in a paper on the deep web published in The Journal of Electronic Publishing, mentioned that Jill Ellsworth used the term Invisible Web in 1994 to refer to websites that were not registered with any search engine. Bergman cited a January 1996 article by Frank Garcia: It would be a site that's possibly reasonably designed, but they didn't bother to register it with any of the search engines. So, no one can find them! You're hidden. I call that the invisible Web. Another early use of the term Invisible Web was by Bruce Mount and Matthew B. Koll of Personal Library Software, in a description of the No. 1 Deep Web program found in a December 1996 press release. The first use of the specific term deep web, now generally accepted, occurred in the aforementioned 2001 Bergman study.

Indexing methods

Methods that prevent web pages from being indexed by traditional search engines may be categorized as one or more of the following:

Content types

While it is not always possible to discover directly a specific web server's content so that it may be indexed, a site potentially can be accessed indirectly (due to computer vulnerabilities). To discover content on the web, search engines use web crawlers that follow hyperlinks through known protocol virtual port numbers. This technique is ideal for discovering content on the surface web but is often ineffective at finding deep web content. For example, these crawlers do not attempt to find dynamic pages that are the result of database queries due to the indeterminate number of queries that are possible. It has been noted that this can be overcome (partially) by providing links to query results, but this could unintentionally inflate the popularity of a site of the deep web. DeepPeep, Intute, Deep Web Technologies, Scirus, and Ahmia.fi are a few search engines that have accessed the deep web. Intute ran out of funding and is now a temporary static archive as of July 2011. Scirus retired near the end of January 2013. Researchers have been exploring how the deep web can be crawled in an automatic fashion, including content that can be accessed only by special software such as Tor. In 2001, Sriram Raghavan and Hector Garcia-Molina (Stanford Computer Science Department, Stanford University) presented an architectural model for a hidden-Web crawler that used important terms provided by users or collected from the query interfaces to query a Web form and crawl the Deep Web content. Alexandros Ntoulas, Petros Zerfos, and Junghoo Cho of UCLA created a hidden-Web crawler that automatically generated meaningful queries to issue against search forms. Several form query languages (e.g., DEQUEL ) have been proposed that, besides issuing a query, also allow extraction of structured data from result pages. Another effort is DeepPeep, a project of the University of Utah sponsored by the National Science Foundation, which gathered hidden-web sources (web forms) in different domains based on novel focused crawler techniques. Commercial search engines have begun exploring alternative methods to crawl the deep web. The Sitemap Protocol (first developed, and introduced by Google in 2005) and OAI-PMH are mechanisms that allow search engines and other interested parties to discover deep web resources on particular web servers. Both mechanisms allow web servers to advertise the URLs that are accessible on them, thereby allowing automatic discovery of resources that are not linked directly to the surface web. Google's deep web surfacing system computes submissions for each HTML form and adds the resulting HTML pages into the Google search engine index. The surfaced results account for a thousand queries per second to deep web content. In this system, the pre-computation of submissions is done using three algorithms: In 2008, to facilitate users of Tor hidden services in their access and search of a hidden .onion suffix, Aaron Swartz designed Tor2web—a proxy application able to provide access by means of common web browsers. Using this application, deep web links appear as a random sequence of letters followed by the .onion top-level domain.