MUD (also referred to as MUD1, to distinguish from its successor, MUD2)  is the oldest virtual world1 in existence. Originally developed in and around 1978 at Essex University, England, MUD ran for many years on the University's computers.

Due in part to a fortuitous coincidence (MUD was written for the same DECSystem-10 computing platform that CompuServe used for its information service) MUD was licensed by CompuServe in the mid-1980s where it ran as a popular game until late 1999. It was eventually retired along with other software during CompuServe's Y2K cleanup efforts. (For a retrospective, please see Memory Lane, by Veni the former BL wizard.)

The version you see here is a straightforward port of the original MUD code base (1985 version) to modern 32-bit platforms (the server runs on both Windows and Linux.) The author of this port2 is Viktor Toth (also known as MrSpock the wizard to players of British Legends, or Viktor the arch-wizard at MUD2.COM.) The original code3 remains the copyrighted property of Roy Trubshaw and Richard Bartle, and is used with permission.

1I used to call MUD "the oldest multi-user game" but, as a critical reader of this Web page pointed out, that is simply not true. Multi-user games have been around for nearly two decades by the time MUD was born. (That is, multi-user computer games. Other multi-user games have been around a lot longer, if Egyptian tomb paintings are to be believed!)
2This MUD1 implementation has been deposited with the Stanford library for historical purposes (reading-room access only)
3The original MUD1 source code is now available on GitHub