In the wee hours of the morning earlier this week, Mainz, Germany, was covered in a strange fog unlike anything residents had ever seen before. That day started out as a sunny one, so the bizarre haze seemed out of place.
 
That was when city officials came racing to the scene and told people to remain in their homes. Something dangerous was afoot.
 
A truck carrying a tank of carbon dioxide was parked in the city that day.

The concentration of carbon dioxide was high enough to knock people out, or even kill them.

With the help of gas masks, city workers were able to remove much of the gas.