The Pitch Drop Experiment at the University of Queensland and its current custodian, John Mainstone. This picture was taken in 1990, two years into the life of the eighth drop. / Image Source: John Mainstone

The world's longest-running scientific experiment is about to drop into the headlines. For over 80 years, two physicists at the University of Queensland have been standing guard over the Pitch Drop Experiment. Now, the decade-awaited moment of truth is bearing down: a drop is about to fall.In 1927, the first physics professor at the University of Queensland, Thomas Parnell, sought to demonstrate that some materials exhibit seemingly contradictory properties. Once used to seal the bottom of boats, tar pitch feels solid at room temperature and shatters like glass under a hammer blow. But, as Parnell has undeniably demonstrated, pitch is actually a very, very viscous fluid.So one day, Parnell placed a block of tar pitch in a sealed funnel. Three years later, the pitch had settled into the bottom of the funnel and in 1930 the bottom of the funnel stem was cut.The first drop fell in December, 1938 It is something of an overstatement to say that the pitch has been dripping ever since . In 86 years, a total of eight drops have dripped, with about a decade between drop falls. Now, the ninth is about to drop...If you're lucky, you may see what no one has seen before—no one has ever seen the drop fall.John Mainstone, a physics professor at the University of Queensland and the experiment's current custodian, missed the last two drops by pure bad luck . Awaiting the eighth drip from a business trip, Mainstone secured a video surveillance system to trail the elusive drop. Alas, the video feed failed precisely during the fall of the eighth pitch-drop. Listen to John Mainstone tell the story of the missed adventure on last week's Radio Lab