In the early 1960s, the location of ocean earthquakes was poor, with errors of up to 300 kilometres. That changed when the location of nuclear tests became a political priority. A new network of Press-Ewing seismometers came into use in 1963; they were well calibrated and had good timing, bringing location errors down to about 50 km and for the first time revealing the earthquake mechanism. “And the first discovery was that all the patterns corresponded to movement of faults,” says McKenzie. “Overnight the whole notion of earthquakes being caused by volcanic explosions underground disappeared.” The new technology showed earthquakes clustered along the mid-ocean ridges and the near-shore trenches.