Pyroclastic surges are lethal hazards from volcanoes that exhibit enormous destructiveness through large dynamic pressures of 100–102 kilopascal inside flows that are capable of obliterating reinforced buildings. These hot, fast and turbulent gas-and-ash clouds create far greater devastation to life and infrastructure than can currently be explained scientifically. But why are they so destructive? Until now, the hostile conditions inside pyroclastic surges have precluded measurements of their inner workings and therefore their hazards remain poorly mitigated globally.