Zork: The Undiscovered Underground (or ZUU for short) is an interactive fiction computer game written by former Infocom implementors Marc Blank and Mike Berlyn . The game was released by Activision in 1997 for free to coincide with the release of Zork Grand Inquisitor. ZUU can be seen as having two goals: promotion of the commercial title Zork Grand Inquisitor, and an attempt to reconcile with die-hard Infocom fans who may have harbored resentment against Activision for their role in buying Infocom in 1985 and subsequent closure of the company in 1989.

Zork: The Undiscovered Underground, a prequel to Zork Grand Inquisitor, is set in the year 1066 GUE. Your character is a private in the Inquisition Guard, tasked with exploring a recently-discovered area of the Great Underground Empire. The game merges classic "Zorkian" references such as grues and zorkmids with elements created for the newer game, such as the Grand Inquisitor.