Only 4 percent of Internet content is visible to the general public, and the other 96 percent is hidden behind the deep web. Deep web content is believed to be about 500 times bigger than normal search content, and it mostly goes unnoticed by regular search engines. The reason Google is not picking up these data, or why deep web content does not get indexed is not a hidden secret. It is mainly that these businesses are either illegal or bad for the society at large. Everything is behind the veil and stays hidden internally on the Tor network.”]
Source: https://hackercombat.com/the-best-10-deep-web-search-engines/