The longest running lab experiment in history, according to the Guinness Book of World Records, began in 1927. Professor Thomas Parnell set out to prove that pitch, in this case natural asphalt, which shatters like glass when you hit it with a hammer, is in fact an extremely slow-moving liquid.
Since the experiment’s inception, there have been nine drops. Parnell survived to see two of them. The first seven drops were, on average, about 8 years apart. The last two were spread out by 13 years.
Until the last drop, the ninth. on April 24, 2014, no one had ever directly witnessed a drop. The next one is due in 14 years.