A few years ago, it was not uncommon to hear search people talk about The Dark Web (also known as Dark Net or the Deep Web). Basically, the Dark Web is made up pages or files that are unreachable by search engines. Examples of this sort of content include information that sits behind login, or pages without inbound links. Some estimates put the Dark Web at about 400 times the size of the Surface Web (content indexed by search engines).
Google has invested tremendous resources in lighting up, or indexing these hidden pieces of content. An example of their efforts was their 2008 announcement that they could now index the contents of .PDF files, a “feat that requires and an immense amount of processing power.”