The seismic waves began 15 miles off the shores of Mayotte in early 2018, a French island between Africa and the northern tip of Madagascar producing a wave that rippled across the Earth.
Twitter user matarikipax spotted the strange motion on U.S. Geological Survey graphs. He noticed a “most odd and unusual seismic signal” on data from Kilimambogo, Kenya; Lusaka, Zambia, Mount Furi, Ethiopia; San Pablo, Spain; and Wellington, New Zealand.
The signals, National Geographic reported, resembled the long-period surface waves that rumble out from earthquakes alongside other higher-frequency waves. But there was no major earthquake that morning to set off the ripple.
Stranger still, the waves were monochromatic, stripped of the fuzzy noise that multiple different frequencies create.
“I don’t think I’ve seen anything like it,” Göran Ekström, a seismologist at Columbia University, told National Geographic.