I almost ran over the Greek alphabet

The other day I needed to print some of my reading material for the week and decided to zip over to the main campus. The moment several Greek letters stepped out onto the path in front of me I probably should have realized that something was going on.
I recieved a definite second clue when I came up to the Student Activities center. The plaza in front of it was fuller than I had ever seen it - and there were even more (different) Greek letters running around. That, and on a small stage three people were standing yelling out various combinations of Greek letters (had I not known better at this point I probably would have assumed they were going through physical equations!) to which from somewhere on the quad several people would start yelling at the top of their voices...
Suddenly, it made sense why Fraternities and Sororities are called the Greek system :-D