A pyroclastic flow is the wall of death that entombed the ancient Roman city of Pompeii. These blackened, billowing clouds of 700°C gas, ash, and rock thunder down the slopes of erupting volcanoes at speeds up to 725 kilometers per hour, incinerating and demolishing most everything in their path. Now, a new study finds pyroclastic flows owe their lethal speed and range to a cushion of air that forms beneath them, National Geographic reports. This layer of air allows the ashen flow to float above the landscape, cutting down on friction and increasing the plume’s speed and distance, the researchers report in Nature Geoscience. The findings could help volcanologists more accurately predict which cities are at risk of being engulfed following volcanic eruptions.