Back in September 2023, scientists monitoring seismic activity across the world were all met with a mysterious signal making its way throughout the whole of the planet. The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

"We were baffled – the signal was unlike any previously recorded," Stephen Hicks, Research Fellow in Computational Seismology at UCL, and Kristian Svennevig, Senior Researcher at the Geological Survey of Denmark and Greenland, explain in a piece for The Conversation . "Instead of the frequency-rich rumble typical of earthquakes, this was a monotonous hum, containing only a single vibration frequency. Even more puzzling was that the signal kept going for nine days."

The signal was detected from the Arctic to Antarctica,

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