The extreme carbon dioxide outburst at the Menzengraben potash mine 7 July 1953

► A forceful accidental outburst of 1100-3900 tonnes of CO2 is analysed. ► The CO2 accumulated in a valley and led to asphyxiation fatalities. ► Dispersion modelling predicts that the high-momentum release would dilute safely. ► Predicted concentration contours cannot explain the asphyxiation harm observed.