What can be considered one of the longest running, single experiments finally has some closure after 69 years. The exiting moment that brought it to a close? A drop of "pitch-tar" finally fell -- and someone saw it.

That's right, the experiment at Trinity College Dublin was set up in 1944 to show the high viscosity of pitch/bituman/asphalt (the substance goes by many names), according to the college's website.

A drop of pitch-tar gets ready to fall. This only happens about once a decade and is the first time it has ever been witnessed. (Image: YouTube screenshot)

The substance might appear solid at room temperature, but it is actually moving very, very slowly. According to the institution, it didn't take nearly seven decades for a tar droplet to form and drip from the funnel, but no one had ever witnessed it happen since the experiment was running. A drop happens about once a every 10 years.

This spring when it looked like a drop was getting close to falling, Professor Shane Bergin wasn't going to let anyone miss it, putting a webcam on it. Last week on July 11, the drop finally fell.

“People love this experiment because it gets to the heart of what good science is all about – curiosity," Bergin said in a statement. Over these past few months, there has been constant chat about when the drip would drop. I watched the time lapse video of the pitch drop falling over and over again. I was amazed. This was the first time this phenomenon was ever witnessed!”

Here's a time-lapsed video of the momentous occasion:

Having video evidence of the droplet and knowing how long it took to fall since the last drop, physicists estimate the substance is 2 million times the viscosity of honey.

The University of Queensland holds the Guinness World Record for longest experiment for a similar set up that has been running since 1927, but no one has yet witnessed it.

(H/T: Nature)

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