For the study, researchers from MIT and Northeastern University, in the United States, and the Technion and Ben-Gurion University, in Israel, had access to three years of anonymized cellphone metadata for an unidentified “west European country.” A total of 12 billion cellphone calls over that three-year span constituted the raw data on which the group would be testing their theories.

The study began with the observation that no cellular provider could possibly monitor and analyze metadata over its entire network in real time. But real-time analysis is key to discovering emergency events. So sampling tiny subsets of metadata from the network—without ever analyzing the content of the calls and text messages—was really the only way to proceed.