Furthermore, a resource may reside in the surface Web, but it has not yet been found by a search engine.

The collection of reachable pages defines the Surface Web.

Most of the work of classifying search results has been in categorizing the surface Web by topic.

If a search engine provides a backlink for a resource, one may assume that the resource is in the surface Web.

Science.gov allows users to search the surface Web as well as the deep Web, where traditional search engines cannot go.

This technique is ideal for discovering resources on the surface Web but is often ineffective at finding deep Web resources.

Automatically determining if a Web resource is a member of the surface Web or the deep Web is difficult.

A. labyrinthica build flat surface webs connected to funnel-shaped retreats similar to labyrinths, which are typically constructed between low lying grass and vegetation.

While technology has increased the availability of scientific and technical information on the surface Web, important scientific information embedded in databases has not been easily searchable.

BrightPlanet prefers the term "deep Web," an online frontier that it estimates may be 500 times larger than the surface Web that search engines try to cover.

Even if a backlink does exist, there is no way to determine if the resource providing the link is itself in the surface Web without crawling all of the Web.

Therefore, if we have an arbitrary resource, we cannot know for sure if the resource resides in the surface Web or deep Web without a complete crawl of the Web.

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.

The surface Web (also known as the Clearnet, the visible Web or indexable Web) is that portion of the World Wide Web that is indexable by conventional search engines.

The Deep Web (also called the Deepnet, the Invisible Web, the Undernet or the hidden Web) is World Wide Web content that is not part of the Surface Web, which is indexed by standard search engines.

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.