The deep Web refers to content of the Internet that is not indexed by search engine spiders and bots. As of 2014, the deep Web is reported to contain roughly 96 percent of the world's Internet content and is only accessible anonymously.
The deep Web requires the use of a special type of anonymous browser, the most commonly used being “The Onion Router,” or TOR. TOR finds information unavailable through surface browsers, such as limited-access content and pages that are not linked to other pages. The deep Web is legal to use, but due to the anonymous nature, it is widely used for illegal activity.