Whistlers were first occasionally heard at the end of the 19th century on long-distance telephone lines. They are electromagnetic emissions produced in the atmosphere, but their cause is still partly unclear. They originate from thunderstorms or meteorites, or even after earthquakes. Once produced, the sounds travel along closed magnetic field lines from one hemisphere to the other. Studying them can yield information about the Earth's atmosphere, ionosphere, and magnetosphere up to very long distances. This one is a lightning strike recorded by Cluster (courtesy of Prof. D. Gurnett, University of Iowa).