Hello, in relation to the term Flash Mob, back in the 60's we did what was called "A happening"
As art students at Ohio State University, we secretly planned events that were unannounced that were timed to take place in the hall during class changes. We created environments such as a beach scene with real sand (on plastic sheeting) waves (a fan blowing under plastic to make it move). People (in the winter) were dressed in swim suits, putting on tanning oil, listening to portable radios (boom boxes) throwing beachballs, etc. acting oblivious to the crowds that would gather to watch.