An Arctic blast sweeping Canada and parts of the United States this week closed schools, grounded jets, killed at least four people, and even shook the ground.
"What the hell was that?" screamed a startled woman at a loud crack produced by a "frost quake" in Ottawa last night.
Frost quakes, or cryoseisms, occur when frozen water-saturated ground cracks in extreme cold.
The University of Toronto Climate Lab tracked dozens of such events, mostly in Ottawa and Montreal regions, but also in a few US states.
Temperatures plunged as low as minus 40 degrees Celsius /Farenheit with a windchill on today.
The cold is considered dangerous at this level, freezing exposed skin within minutes.
Officials in both countries urged people to stay indoors and not to travel unless absolutely necessary. Pet owners were also told not to let dogs or cats out.
People who must venture out were told to dress in layers covering all skin, to stay dry, to seek shelter and not drink alcohol which gives people a false sense of warmth.
Temperatures dropped at the beginning of the week and the cold is forecast to linger until at least Saturday.
In Ottawa, most bureaucrats stayed home after temperatures plunged overnight following several days of snow and freezing rain.
Ice on streets sent cars sliding into ditches as road crews worked around the clock to clear snow and salt roadways with little success.
