A huge amount of carbon dioxide (CO2) leaked from a transport truck in central Mainz, Germany, on Tuesday, immobilising traffic around the centre of the city and forcing the closure of the Christmas market.
Police said that the gas had leaked from the truck parked near to a champagne producer's building.
The thick white clouds of heavier-than-air gas lay close to the ground of surrounding streets, leaving some buses, trains and cars unable to move due to the low visibility.
Local people were told not to go into their basements and to avoid the ground floor if possible, as the colourless, odourless gas can suffocate people if it displaces all the oxygen in a room.
The driver of the truck was slightly hurt by the freezing-cold gas and treated as an outpatient at the hospital.
It was not immediately clear whether a technical fault or human error led to the leak. Police have impounded the truck and are investigating.