Most of us use the internet daily without realizing we are able to access only a very small part of online information and websites.
The numerous online sites that are available only with restricted passwords are often referred to as the “dark net”, in reference to its clandestine utilization by organizations of both “good guys” and “bad guys”. Criminal websites are available there (you can buy, e.g., drugs, munitions, and slaves), as are websites used by political activists, whistle blowers, and intelligence professionals.
Originally developed by the US Navy, the “dark net” is now a controversial technology within government circles. The NSA and FBI want to either shut it down or to be able to fully monitor it (by limiting encryption) while the DOD, Department of State, and others value it because of its usefulness.
Even if you are not a covert operator, you will likely find this presentation quite informative and interesting.