Zork

A bunch of university kids at MIT (soon to be the guiding lights behind Infocom) played the pioneering cave-crawling treasure-hunting text adventure ADVENTURE and thought "Hey, we can improve on that!" Around June of 1977 they pooled their collected creative juices and cranked out a sassy successor in MDL on a PDP-10. It didn't have a name, so they just used a hacker nonsense word in place of one temporarily -- "Zork."

This game presented the next in a long line of treasure-hunting protagonists with a vast Great Underground Empire to explore -- provided their lantern didn't run out of juice -- and whimsical obstacles among the Flatheadian ruins forced players to employ lateral thinking to collect wondrous treasures worth thousands of zorkmids... and deposit them in the trophy case back aboveground. If this sounds familiar, it's no coincidence -- Zork/Dungeon is the primordial clay from which the first three games of the Zork series were fashioned, following some extensive refitting of set pieces -- due to the game's enormous size of one megabyte, making it commercially available for home microcomputers, with their limited memory and storage capabilities, was not considered a viable plan in 1979... until they came up with the idea of segmenting the giant dungeon up.

(Ports of this game also exist for VMS, the Acorn Archimedes, and the Z-code, TADS and GLK software platforms.)