In Göttingen, Germany, there's a four-tonne steel ball that can be raised up a 14-metre tower -- and then dropped in less than two seconds, crashing back to earth. It makes tiny, artificial earthquakes: here's why.
Three things I had to cut out of this video, because they didn't quite fit into the story or because I couldn't film them:
The reason the steel ball survived two world wars is because the university's records listed it by use as a "rock-ball", not by composition as a "steel ball" - so no-one melted it down for weaponry.
The observatory team refill that pit every year to make the ground flat, and the ball just digs a hole again. The rock's just being compressed underneath. They joke that, somewhere in Australia, there's a slowly growing hill...
And finally, the ground steams for a little while after the ball hits: it gets rather warm.
Watch the video here:
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