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