The main rotunda. The first afternoon I was there, there was a picnic for all the incoming freshmen. The chancellor gave a speech while standing in front of the columns. During his speech, some of the upperclassmen lowered a large, life-size cow from the top of the rotunda.
This is referred to as a "hack" - an ingenious practical joke, often done in a public setting. For more on hacks, look at: http://www-tech.mit.edu/V118/N3/ctbp.3n.html.