Like a glacier in the sea, only a fraction of the World Wide Web is visible by search engines.
Google has indexed trillions of pages on the web, but the monolithic search engine really only scrapes the surface. Though they’re constantly crawling the web and adding new pages, search engines can only access about 1% of the entire World Wide Web.
The other 99% is called the Deep Web.
But the Deep Web isn’t as nefarious as it sounds. Yes, it does include secretive and sometimes illegal sites that can only be accessed on the TOR network. But the majority of Deep Web pages are much more mundane.
Many pages on the Deep Web aren’t indexed because search engine crawlers don’t use websites the same way people do. For example, dynamic web pages created on-the-fly for users don’t show up in Google. If you use your favorite online store’s search engine, the website creates a unique, original page in order to show you your search results. That page created just for you is part of the Deep Web.