German scientist Otto von Guericke conducted a memorable experiment on May 8, 1654: He connected two hemispheres, sealed their rims together, and drew out the air between them using a pump of his own devising. The resulting vacuum was so strong that 30 horses could not pull them apart.

At the time the experiment was seen as a strike against Aristotle’s dictum that nature abhors a vacuum. It’s repeated today as a dramatic demonstration of the power of atmospheric pressure.