Undernet is an Internet Relay Chat (IRC) network set up in the early 1990s and is now one of the largest of such networks in the world. This international network is estimated to serve more than one million users each week and connects servers in dozens of countries.
Techopedia explains Undernet
As an IRC channel, Undernet is an accessible means of real-time messaging for users. It supports all major operating systems (OS) and is searchable through specific search engine technology. One of the biggest issues with Undernet and other IRC channels is the way user habits and trends change over time. As new social media technologies emerge on the scene, many have asked the question "do people still use IRC?"
A relatively new way to break down the use of Undernet and other IRC channels - compared to the use of technologies like Facebook, Yahoo messaging or prominent IM tools - is that IRC users tend to be more savvy about the underlying technology. It generally takes more work to access and set up IRC connections than it does to enable more familiar interfaces for real-time communications.