Many people don’t fathom just how much information is available on the internet but NOT indexed by Google or other search engines who primarily crawl just what is easily available. The dark/deep web/darknets are largely ignored by anyone not in the know, which is unfortunate.
PC Pro has an interesting article on this which is worth taking a look at. It spins a bit of an ominous view of FreeNet and TOR, both of which have pros and cons to their use, and doesn’t go into enough detail regarding the deep web and what sort of institutional holdings make up the bulk of what you can find but it is a good read nonetheless.
MakeUseOf (a must-read for anyone with the slightest technological bent) has a decent but hardly exhaustive list of search engines that crawl the deep web. For anyone interested in further exploring deep web resources I recommend taking a look at Laguardia Community College’s Beyond Google page as a good starting point.