On a corner of the University of Zurich’s theological school at Grossmünster sits a boulder. Though the church structure is nearly 1,000 years old, this boulder only came to be there some 350 years ago, when a nearby tower exploded after being struck by lightning.

The Geissturm was once part of the fortifications for the city of Zurich. It was a tower located on the old town wall on a hill called Winkelwiese. Because of its distance from the inhabited areas of town, the tower was used as a gunpowder depot starting in the 16th century (hence its other name, Pulverturm, or powder tower).

On June 10, 1652,  a bolt of lightning struck the Geissturm, igniting a mighty explosion involving 400 hundredweights of powder. The tower and parts of the wall that surrounded it were destroyed, and seven people were killed in the disaster. Thrown by the force of the explosion, the stones that made up the tower and the wall were scattered around the surrounding neighborhood.

One particularly large boulder, weighing about 1,800 kilograms (4,000 pounds), was hurled over 230 meters (750 feet) and landed at this spot next to the Grossmünster, where it remains to this day.