system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the deep internet work? 

How does one get there and what happens on all of the levels below the surface which I barely scratch?
gpt
The "deep web" or "deep internet" is mostly an urban legend for people who don't know very much about computer networking. The term "deep web" originally meant the part of the Internet not indexed by public search engines. 

This includes the internal networks of companies, universities and other institutions, as well as private data on public websites. For example, you will only be able to access a tiny fraction of data on facebook, making most of facebook part of the deep web.

The term in this sense was created when the shift from static web (HTML documents on a server) pages to dynamically generated web pages (generated on request from information in a data base by a computer program) happened. For example, the Google site offers a wealth of information, but if you don't enter a search term, you won't find any of it. This makes Google search results pages part of the dark web by that original definition. To access all of Google, you would have to enter all possible search terms, which is not feasible. This didn't use to be like that, in the beginning of the internet, it was possible to see pretty much all content there was. The term "deep web" was created to state this change of internet structure from mostly-available to mostly-protected.

Apart from that, there are also encryption services like tor or i2p, that allow users to visit and host websites anonymously. They are mainly used by technology enthusiasts, but also by criminals, whistleblowers and political dissidents. This is because only those people need such strong protection of their information. As you might imagine, sites where you can buy and sell stolen credit card information, drugs, child and other illegal porn, and similar can be found on such networks. These sites are interested in visitors, just like any site, so they will advertise on places like the hidden wiki, an indexing service on tor which is publically accessible.

Apart from the two categories mentioned above, there also exist darknets. A darknet is simply a network of people who privately share information of any kind, using any networking technology. Darknets become dark because they do not advertise publically, but are in no other way affiliated with illegal or immoral actions. A group of people with a private server to share their vacation pictures - who need a password to access the server - is a darknet, for example.

In conclusion, the "deep web" is a somewhat technical term that got turned into a meme by people who do not understand the workings of the internet. Some people (well, shitty "info-graphics") nowadays even associate completely public sites like 4chan with the deep web, which makes no sense at all.