At least 11 people have died during the icy grip of the so-called polar vortex that has plunged some areas of the U.S. into Antarctic-level temperatures this week.

How low did it go?

Many Midwestern cities are colder than some of the coldest places on earth, including Siberia, the Arctic, Antarctica, Alaska and "even other planets," The Verge reported. Earlier this week a high of -5 degrees was reported on Mars.

A record-low temperature of -21 was recorded at O'Hare International Airport in Chicago on Thursday. The previous record low on Jan. 31 was -12 in 1985. The all-time record low of any day in Chicago is a temperature of -27.

The freezing weather cancelled nearly 60 percent of flights on Wednesday at O'Hare International and Midway International airports.

Also, a state of emergency was declared in Wisconsin, Michigan, Illinois, Mississippi and Alabama.

On Wednesday, snow fell across the Great Lakes region and into New England. Up to 24 inches was forecast in Wisconsin; six inches of snow was expected in Illinois. By the time the weekend arrives, about 20 million people in the U.S. will experience temperatures of at least -18 degrees.

Weather conditions are so cold that it's possible to get frostbite from being outside for as little as 5 to 10 minutes, according to published reports.

Grand Forks, North Dakota, has seen one of the lowest wind chills so far, a -54 degrees fahrenheit reading was recorded Wednesday morning.

What were the fatalities?

Fatalities linked to the weather reported by various media outlets include deaths from a number of diverse weather-related causes, including: