The pitch-drop experiment has been running since 1927

In 1927, Thomas Parnell, the first professor of physics at the University of Queensland, set out to prove a strange point: some things that look perfectly solid are actually fluids creeping along too slowly to see. He melted pitch — the tar-like residue left from distilling coal or wood — poured it into a sealed glass funnel, and let it settle for three years before snipping the stem open in 1930.

Then he waited. Pitch feels brittle enough to shatter with a hammer, yet it is roughly 100 billion times more viscous than water, so it oozes downward almost imperceptibly.

It took eight years for the very first drop to fall — and no one was watching when it did.

Nearly a century on, only nine drops have dropped, the latest in 2014. Astonishingly, no person has ever directly witnessed one land; custodian John Mainstone watched the experiment for 52 years and died without seeing a fall. Recognised by Guinness World Records as the longest-running laboratory experiment, it earned an Ig Nobel Prize in 2005.