The experiment, meant to show that pitch, although considered a solid, is actually one of the world's most viscous liquids, was set up at Trinity College Dublin in 1944.
"No one has ever seen a drop fall anywhere in the world," Trinity's Shane Bergin, whose Web cam caught the event on July 11, told NewScientist.com. "It's one of the oldest experiments -- an oddity, a curiosity."
The original version of the experiment, set up at the University of Queensland in Australia, has been running since 1930, but various problems have prevented Australian researchers from ever witnessing the actual fall of a drop.
In 1927 Queensland physicist Thomas Parnell poured a blob of pitch into a glass funnel with a sealed stem. After the pitch was allowed to cool and settle for 3 years, the stem was cut to allow the pitch to drip out and fall.
Gravity has since pulled down a viscous drop about once a decade, but never when a witness was present.
No one at Trinity College remembers who set up their version of the experiment, which sat on a shelf mostly ignored for years.
Bergin set up a Web cam last year so that anyone around the world could join the vigil.
The video shows the drop falling in two stages, Bergin said. "There was one heavy fall, and then there was a tiny thread left that was still connected. That broke while I was there."
Researchers say the experiments demonstrate pitch is 2 million times more viscous than honey, and 2 billion times more viscous than water.