The game was inspired by Colossal Adventure. The first version was released in 1977, and work continued until 1979. Along the way, the game changed name from Dungeon to Zork to avoid copyright issues with a certain RPG company.
This game later formed the basis of Zork I-III, but the latter two in particular were greatly extended.
Read the detailed (and very entertaining) story about the game's creation.
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Was the subject of a lot of collective, competitive adventuring back in the day, particularly by uni students. Remains a lot of fun, but it's aimed at long-term play. Some say that the segmentation it later underwent to become the three commercial Zorks did not do it any favours.