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The Deep Web (also called the Deepnet,[1] Invisible Web,[2] or Hidden Web[3]) is World Wide Web content that is not part of the Surface Web, which is indexed by standard search engines. It should not be confused with the dark Internet, the computers that can no longer be reached via the Internet, or with a Darknet distributed filesharing network, which could be classified as a smaller part of the Deep Web. There is concern that the deep web can be used for serious criminal activity.[4]

Mike Bergman, founder of BrightPlanet and credited with coining the phrase,[5] said that searching on the Internet today can be compared to dragging a net across the surface of the ocean: a great deal may be caught in the net, but there is a wealth of information that is deep and therefore missed.[6] Most of the Web's information is buried far down on dynamically generated sites, and standard search engines do not find it. Traditional search engines cannot "see" or retrieve content in the deep Web—those pages do not exist until they are created dynamically as the result of a specific search. As of 2001, the deep Web was several orders of magnitude larger than the surface Web.[7]


Estimates based on extrapolations from a study done at University of California, Berkeley in 2001[7] speculate that the deep Web consists of about 7.5 petabytes. More accurate estimates are available for the number of resources in the deep Web: research of He et al. detected around 300,000 deep web sites in the entire Web in 2004,[8] and, according to Shestakov, around 14,000 deep web sites existed in the Russian part of the Web in 2006.[9]


Bergman, in a seminal 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.[7] Bergman cited a January 1996 article by Frank Garcia:[10]

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 @1 deep Web tool found in a December 1996 press release.[11]

The first use of the specific term Deep Web, now generally accepted, occurred in the aforementioned 2001 Bergman study.[7]

Deep resources[edit]

Deep Web resources may be classified into one or more of the following categories:

  • Dynamic content: dynamic pages which are returned in response to a submitted query or accessed only through a form, especially if open-domain input elements (such as text fields) are used; such fields are hard to navigate without domain knowledge.
  • Unlinked content: pages which are not linked to by other pages, which may prevent Web crawling programs from accessing the content. This content is referred to as pages without backlinks (or inlinks).
  • Private Web: sites that require registration and login (password-protected resources).
  • Contextual Web: pages with content varying for different access contexts (e.g., ranges of client IP addresses or previous navigation sequence).
  • Limited access content: sites that limit access to their pages in a technical way (e.g., using the Robots Exclusion Standard, CAPTCHAs, or no-cache Pragma HTTP headers which prohibit search engines from browsing them and creating cached copies.[12])
  • Scripted content: pages that are only accessible through links produced by JavaScript as well as content dynamically downloaded from Web servers via Flash or Ajax solutions.
  • Non-HTML/text content: textual content encoded in multimedia (image or video) files or specific file formats not handled by search engines.


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 resources on the surface Web but is often ineffective at finding deep Web resources. 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.[5] It has been noted that this can be (partially) overcome by providing links to query results, but this could unintentionally inflate the popularity for a member of the deep Web.

In 2005, Yahoo! made a small part of the deep Web searchable by releasing Yahoo! Subscriptions.[citation needed] This search engine searches through a few subscription-only Web sites.[citation needed] Some subscription websites display their full content to search engine robots so they will show up in user searches, but then show users a login or subscription page when they click a link from the search engine results page.[citation needed]

DeepPeep, Intute, Deep Web Technologies, and Scirus 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.[13] Scirus retired near the end of January, 2013.[14] DeepPeep retired sometime in early 2014.[citation needed]

Some so-called hidden services can be accessed only through an onion router such as Tor (anonymity network) which allows the user to access .onion web pages.

Crawling the Deep Web[edit]

Researchers have been exploring how the Deep Web can be crawled in an automatic fashion. In 2001, Sriram Raghavan and Hector Garcia-Molina (Stanford Computer Science Department, Stanford University)[15][16] presented an architectural model for a hidden-Web crawler that used key terms provided by users or collected from the query interfaces to query a Web form and crawl the Deep Web resources. Alexandros Ntoulas, Petros Zerfos, and Junghoo Cho of UCLA created a hidden-Web crawler that automatically generated meaningful queries to issue against search forms.[17] Several form query languages (e.g., DEQUEL[18]) have been proposed that, besides issuing a query, also allow to extract 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.[19][20]

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 mod oai 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 directly linked to the surface Web. Google's deep Web surfacing system pre-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.[21] In this system, the pre-computation of submissions is done using three algorithms:

  1. selecting input values for text search inputs that accept keywords,
  2. identifying inputs which accept only values of a specific type (e.g., date), and
  3. selecting a small number of input combinations that generate URLs suitable for inclusion into the Web search index.

Classifying resources[edit]

Automatically determining if a Web resource is a member of the surface Web or the deep Web is difficult. If a resource is indexed by a search engine, it is not necessarily a member of the surface Web, because the resource could have been found using another method (e.g., the Sitemap Protocol, mod_oai, OAIster) instead of traditional crawling. If a search engine provides a backlink for a resource, one may assume that the resource is in the surface Web. Unfortunately, search engines do not always provide all backlinks to resources. Furthermore, a resource may reside in the surface Web even though it has yet to be found by a search engine.

Most of the work of classifying search results has been in categorizing the surface Web by topic. For classification of deep Web resources, Ipeirotis et al.[22] presented an algorithm that classifies a deep Web site into the category that generates the largest number of hits for some carefully selected, topically-focused queries. Deep Web directories under development include OAIster at the University of Michigan, Intute at the University of Manchester, Infomine[23] at the University of California at Riverside, and DirectSearch (by Gary Price). This classification poses a challenge while searching the deep Web whereby two levels of categorization are required. The first level is to categorize sites into vertical topics (e.g., health, travel, automobiles) and sub-topics according to the nature of the content underlying their databases.

The more difficult challenge is to categorize and map the information extracted from multiple deep Web sources according to end-user needs. Deep Web search reports cannot display URLs like traditional search reports. End users expect their search tools to not only find what they are looking for special, but to be intuitive and user-friendly. In order to be meaningful, the search reports have to offer some depth to the nature of content that underlie the sources or else the end-user will be lost in the sea of URLs that do not indicate what content lies beneath them. The format in which search results are to be presented varies widely by the particular topic of the search and the type of content being exposed. The challenge is to find and map similar data elements from multiple disparate sources so that search results may be exposed in a unified format on the search report irrespective of their source.


The lines between search engine content and the deep Web have begun to blur, as search services start to provide access to part or all of once-restricted content. An increasing amount of deep Web content is opening up to free search as publishers and libraries make agreements with large search engines. In the future, deep Web content may be defined less by opportunity for search than by access fees or other types of authentication.[citation needed]

See also[edit]


  1. ^ Hamilton, Nigel. "The Mechanics of a Deep Net Metasearch Engine". doi: Check |doi= value (help). 
  2. ^ Devine, Jane; Egger-Sider, Francine (2004). "Beyond google: the invisible web in the academic library". The Journal of Academic Librarianship. 30 (4): 265–269. Retrieved 2014-02-06.  Unknown parameter |month= ignored (help)
  3. ^ Raghavan, Sriram; Garcia-Molina, Hector (11–14 September 2001). "Crawling the Hidden Web". 27th International Conference on Very Large Data Bases. Rome, Italy. 
  4. ^ The Secret Web: Where Drugs, Porn and Murder Live Online
  5. ^ a b Wright, Alex (2009-02-22). "Exploring a 'Deep Web' That Google Can’t Grasp". The New York Times. Retrieved 2009-02-23. 
  6. ^ Bergman, Michael K (July 2000). The Deep Web: Surfacing Hidden Value (PDF). BrightPlanet LLC. 
  7. ^ a b c d Bergman, Michael K (August 2001). "The Deep Web: Surfacing Hidden Value". The Journal of Electronic Publishing. 7 (1). doi:10.3998/3336451.0007.104. 
  8. ^ He, Bin (May 2007). "Accessing the Deep Web: A Survey". Communications of the ACM (CACM). 50 (2): 94–101. doi:10.1145/1230819.1241670.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  9. ^ Denis Shestakov (2011). "Sampling the National Deep Web". Proceedings of the 22nd International Conference on Database and Expert Systems Applications (DEXA) (in Russian). pp. 331–340. Archived from the original (PDF) on September 2, 2011. Retrieved 2011-10-06. 
  10. ^ Garcia, Frank (January 1996). "Business and Marketing on the Internet". Masthead. 15 (1). Archived from the original on 1996-12-05. Retrieved 2009-02-24. 
  11. ^ @1 started with 5.7 terabytes of content, estimated to be 30 times the size of the nascent World Wide Web; PLS was acquired by AOL in 1998 and @1 was abandoned. "PLS introduces AT1, the first 'second generation' Internet search service" (Press release). Personal Library Software. December 1996. Retrieved 2009-02-24. 
  12. ^ pragma:no-cache/cache-control:no-cache "HTTP 1.1: Header Field Definitions (14.32 Pragma)" Check |url= value (help). HTTP — Hypertext Transfer Protocol. World Wide Web Consortium. 1999. Retrieved 2009-02-24. 
  13. ^ "Intute FAQ". Retrieved October 13, 2012. 
  14. ^
  15. ^ Sriram Raghavan (2000). "Crawling the Hidden Web" (PDF). Stanford Digital Libraries Technical Report. Retrieved 2008-12-27.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  16. ^ Raghavan, Sriram (2001). "Crawling the Hidden Web" (PDF). Proceedings of the 27th International Conference on Very Large Data Bases (VLDB). pp. 129–38.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  17. ^ Alexandros, Ntoulas (2005). "Downloading Hidden Web Content" (PDF). UCLA Computer Science. Retrieved 2009-02-24.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  18. ^ Shestakov, Denis (2005). "DEQUE: Querying the Deep Web" (PDF). Data & Knowledge Engineering 52(3): 273–311.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  19. ^ Barbosa, Luciano (2007). "An Adaptive Crawler for Locating Hidden-Web Entry Points" (PDF). WWW Conference 2007. Retrieved 2009-03-20.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  20. ^ Barbosa, Luciano (2005). "Searching for Hidden-Web Databases." (PDF). WebDB 2005. Retrieved 2009-03-20.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  21. ^ Madhavan, Jayant (2008). "Google’s Deep-Web Crawl" (PDF). VLDB Endowment, ACM. Retrieved 2009-04-17.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  22. ^ Ipeirotis, Panagiotis G. (2001). "Probe, Count, and Classify: Categorizing Hidden-Web Databases" (PDF). Proceedings of the 2001 ACM SIGMOD International Conference on Management of Data. pp. 67–78.  Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^

Further reading[edit]

  • Paganini, Pierluigi (May 2012), "What is the Deep Web? A first trip into the abyss", Security Affairs, Napoli, Italy: Security Affairs .
  • Paganini, Pierluigi (Sept 2012), "The good and the bad of the Deep Web", Security Affairs, Napoli, Italy: Security Affairs  Check date values in: |date= (help).
  • Paganini, Pierluigi (Sept 2013), "Traffic Correlation Attacks against Anonymity on Tor", Security Affairs, Napoli, Italy: Security Affairs  Check date values in: |date= (help).
  • Pierluigi Paganini, Richard Amores (Sept 2012), "The Deep Dark Web", The Deep Dark Web, NY, USA: Amazon  Check date values in: |date= (help).
  • Paganini, Pierluigi (Oct 2012), "The Deep Web Part 1: Introduction to the Deep Web and how to wear clothes online!", The Malta Independent Online, Malta: The Malta Independent .
  • Barker, Joe (Jan 2004), "Invisible Web: What it is, Why it exists, How to find it, and its inherent ambiguity", Teaching Library Internet Workshops, Berkeley, CA, USA: UC .

External links[edit]

Category:World Wide Web Category:Information retrieval