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/vad'ing/ (From VAD, a permutation of ADV, i.e. ADVENT, used to avoid a particular admin's continual search-and-destroy sweeps for the game) A leisure-time activity of certain hackers involving the covert exploration of the "secret" parts of large buildings - basements, roofs, freight elevators, maintenance crawlways, steam tunnels, and the like. A few go so far as to learn locksmithing in order to synthesise vadding keys. The verb is "to vad" (compare phreaking; see also hack, sense 9). This term dates from the late 1970s, before which such activity was simply called "hacking"; the older usage is still prevalent at MIT.
Vadding (pronounced /vay'ding/) was also popular CMU, at least as early as 1986. People who did it every night were called the "vaders," possibly after "elevator," which was one of the things they played with, or "invader," or "Darth Vader". This game was usually played along with no-holds-barred hide-and-seek. CMU grad students were the known to pry open the inner doors of elevators between floors to see the graffiti on the inside of the outer doors.
The most extreme and dangerous form of vadding is "elevator rodeo", also known as "elevator surfing", a sport played by wrasslin' down a thousand-pound elevator car with a 3-foot piece of string, and then exploiting this mastery in various stimulating ways (such as elevator hopping, shaft exploration, rat-racing, and the ever-popular drop experiments). Kids, don't try this at home!
See also hobbit.