A series of text adventure games developed by Michael J. Roberts, the creator of TADS (the Text Adventure Development System). The story centers around the Caltech tradition of "ditch day," wherein upperclassmen go away from campus and leave puzzles known as "stacks" behind for the underclassmen.
A somewhat more extensive history by Michael Roberts:
The first few Ditch Days were simple affairs where the seniors really did just ditch classes for the day. The underclassmen were apparently annoyed at being left behind, and they expressed this by breaking into the seniors' rooms and trashing them. At some point, the seniors started trying to barricade their rooms before they left, by stacking desks and other heavy objects against their doors.
That's where the term comes from: the seniors literally built stacks of heavy objects to block entry to their rooms. For their part, the underclassmen weren't deterred; on the contrary, they took the stacks as a challenge. If this had developed into an arms race, the tradition would probably have become too repetitive (and too destructive) to last very long. Happily, the rivalry instead became ritualized: stacks evolved from physical barriers into, essentially, games. Ditch Day today is a highlight of everyone's year at Caltech; underclassmen look forward to it, and seniors take great pride in creating stacks that are original, challenging, and fun.