To discover content on the Web, search engines use web crawlers (algorithmic crawlers) that follow hyperlinks. 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 infinite number of queries that are possible.
One way to explore the deep web is by using human crawlers instead of algorithmic crawlers. In this paradigm, referred to as Web harvesting / Web scraping / Data extraction, a technique wherein human developed customized data extraction solution (often specific to a website) crawls the targeted website. This human-based computation technique to discover the Deep Web has been used by the StumbleUpon service since February 2002.