The Immersion of the High Professor.
Kings Parade in Cambridge was closed to traffic and a procession of hundreds, all in academic garb, ambled (we were instructed not to be formal) down it, through Trinity College and onto the backs. Here the High Professor was presented and stripped down to his underwear behind a curtain of cloaks. The choir chanted a Latin responsary, of which I can still remember this couplet: "In combinationibus stat." (He stands in his combinations.) "Sancta Michaelis designatus est." (Labelled 'St Michael's'.)
The High Professor was then slapped across the face with a fish (according to the handout for the crowd, this was introduced a few hundred years ago, but no one knew why). Finally he was rowed off down the river while the choir sang Superflumina Babyloniis.
According to the handout, this ceremony celebrated the actions of a medieval professor who drowned bathing in the Cam while attempting to set an example of cleanliness to his students. He died at age 68 (I can't remember the exact number, but it was about this) and now the ceremony is enacted every 68 years.
This was all undertaken with the assistance of the police, Trinity College etc. The best bit of all came about 10 years later, at least 100 miles from Cambridge, when we had some people we hadn't seen for ages round for dinner, and one of them told the story of this amazing cermony he had seen while visiting Cambridge as a tourist - he had been one of the huge audience that assumed it was for real.