Shortly after midnight last Sept. 26, a digital seismograph tucked
   inside a shallow well on the Danish island of Bornholm picked up an
   unusual signal. The device, roughly the size and shape of a football
   helmet, registered a pressure wave passing through the rock beneath the
   island. The wave generated a stream of data points that zipped along
   cables to a nearby family’s garage, where a computer snipped the
   signals into chunks representing a few seconds apiece. That data then
   traveled via internet cables strung beneath the Baltic Sea to Copenhage
   The Nord Stream Explosion Remains an Unsolved Mystery Gripping Europe
   [1]https://www.bloomberg.com/news/features/2023-04-20/nord-stream-pipel
   ine-explosion-still-a-mystery-months-later

References

   1. https://www.bloomberg.com/news/features/2023-04-20/nord-stream-pipeline-explosion-still-a-mystery-months-later