The Antarctic Impulsive Transient Antenna (ANITA) is a radio wave detector mounted on a giant balloon at an altitude of 38 kilometres. It is designed to detect the radio waves produced when a high-energy cosmic neutrino smashes into the thick ice below.

But although ANITA has yet to capture this signature, it has found another kind of particle: ultra-high-energy cosmic rays. These are protons and other charged nuclei that zoom in from space with mysteriously high energies.

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