After Hillary Clinton's historic nomination, a sign appeared at Susan B. Anthony's grave.

Because we use water all the time, most of us have an intuitive sense of how long it takes a drop of water to form and fall. More viscous liquids, like oil or shampoo or honey, drop more slowly depending on how thick they are, which can vary depending on concentration, temperature and more. If you've ever tried pouring molasses, you know why it's used as a metaphor for something moving very slowly, but we can easily see a drop of any of those liquids form and fall in a matter of seconds.

But what about the most viscous substance in the world? How long does it take to form a falling drop? A few minutes? An hour? A day?

How about somewhere between 7 and 13 years?

Pitch moves so slowly it can't be seen to be moving with the naked eye until it prepares to drop. Battery for size reference.John Mainstone/University of Queensland

The Pitch Drop Experiment began in 1927 with a scientist who had a hunch. Thomas Parnell, a physicist at the University of Queensland in Australia, believed that tar pitch, which appears to be a solid and shatters like glass when hit with a hammer at room temperature, is actually a liquid. So he set up an experiment that would become the longest-running—and the world's slowest—experiment on Earth to test his hypothesis.

Parnell poured molten pitch it into a funnel shaped container, then let it settle and cool for three years. That was just to get the experiment set up so it could begin. Then he opened a hole at the bottom of the funnel to see how long it would take for the pitch to ooze through it, form a droplet, and drop from its source.

It took eight years for the first drop to fall. Nine years for the second. Those were the only two drops Parnell was alive for before he passed away in 1948.

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In total, there have been nine pitch drops in the University of Queensland experiment. The first seven drops fell between 7 and 9 years apart, but when air conditioning was added to the building after the seventh drop, the amount of time between drops increased significantly. The drops in 2000 and 2014 happened approximately 13 years after the preceding one. (The funnel is set up as a demonstration with no special environmental controls, so the seasons and conditions of the building can easily affect the flow of the pitch.)

The next drop is anticipated to fall sometime in the 2020s.

The first seven drops fell around 8 years apart. Then the building got air conditioning and the intervals changed to around 13 years.RicHard-59

Though Parnell proved his hypothesis well before the first drop even fell, the experiment continued to help scientists study and measure the viscosity of tar pitch. The thickest liquid substance in the world, pitch is estimated to be 2 million times more viscous than honey and 20 billion times the viscosity of water. No wonder it takes so ridiculously long to drop.

One of the most interesting parts of the Pitch Drop Experiment is that in the no one has ever actually witnessed one of the drops falling at the Queensland site. The drops, ironically, happen rather quickly when they do finally happen, and every time there was some odd circumstance that kept anyone from seeing them take place.

The Queensland pitch drop funnel is no longer the only one in existence, however. In 2013, Trinity College in Dublin, Ireland, managed to capture its own pitch drop on camera. You can see how it looks as if nothing is happening right up until the final seconds when it falls.

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Today, however, with the internet and modern technology, it's likely that many people will be able to witness the next drop when it happens. The University of Queensland has set up a livestream of the Pitch Drop Experiment, which you can access here, though watching the pitch move more slowly than the naked eye can detect is about as exciting as watching paint dry.

But one day, within a matter of seconds, it will drop, hopefully with some amount of predictability as to the approximate day at least. How many people are going to be watching a livestream for years, waiting for it to happen?

PoorJohn Mainstone was the custodian of the experiment for 52 years, from 1961 to 2013. Sadly, he never got to witness any of the five drops that took place during his tenure. Neither did Parnell himself with the two that took place while he was alive.

John Mainstone, the second custodian of the Pitch Drop Experiment, with the funnel in 1990.John Mainstone, University of Queensland

Sometimes science is looks like an explosive chemical reaction and sometimes it's a long game of waiting and observing at the speed of nature. And when it comes to pitch dripping through a funnel, the speed of nature is about as slow as it gets.