In 1911 and 1912, an Austrian physicist named Victor Hess took to the sky in a series of risky hot air balloon trips—for science. Down on land, researchers had been registering signals of mysterious energetic particles on their instruments. They didn’t know what the signals were or where they came from. So in progressively thinning air, more than three miles off the ground, Hess performed experiments to figure out if the particles came from above or below.

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