Early Wednesday the temperature there was 19 below zero.
Disruptions caused by the cold will persist, too, including power outages and cancelled flights and trains.
In Detroit, more than two dozen water mains froze.
A jogger runs along the lakefront as temperature hung around -20 degrees on January 30, 2019 in Chicago, Illinois.
The almost unthinkable temperatures caused airline gas lines to freeze and electrical grids to collapse, and they kept much of the northern United States homebound. Temperatures in the region dropped to minus 25 degrees (minus 31 Celsius) Thursday morning but forecasters are predicting a high of minus 2 degrees (minus 16 Celsius). The wind chill temperature, which is a measure of what the air fees like when wind is taken into account, will fall as low as minus-65 in the Upper Midwest. During the early Wednesday morning hours, much of the Dakotas, Minnesota, Wisconsin, and IL are locked down with these "feels like" temperatures in the -40s, -50s, and even -60s (Fahrenheit) as winds howl out of the west at up to 25mph. But students headed back to school in eastern North Dakota, where subzero temperatures were forecast to crawl upward. The deep freeze snapped rail lines, canceled hundreds of flights and strained utilities.
Almost 1,700 flights had been canceled in Chicago by Thursday afternoon.
Chicago's record for coldest temperature was shattered on Wednesday as the polar vortex struck the biggest city in the U.S. Midwest, according to the National Weather Service (NWS). Milwaukee had similar conditions.
Even the state record in IL is in jeopardy - minus-36 degrees set in Congerville on January 5, 1999.
The morning temperature in the Windy City was -22 degrees Fahrenheit (-30 Celsius), which felt like -50 degrees (-46 Celsius) with wind chill. Trains and buses operated with few passengers; engineers set fires along tracks to keep commuter trains moving.
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Others are making crystals from frozen bubbles.
The US Postal Service - known for its commitment to bringing the mail whatever the weather - suspended deliveries in parts of Indiana, Michigan, Illinois, Ohio, Iowa, the Dakotas and Nebraska.
The bitter cold was the result of a split in the polar vortex, a mass of cold air that normally stays bottled up in the Arctic.
At least eight deaths have been linked to the system, including an elderly IL man who was found several hours after he fell trying to get into his home.
He was found unresponsive behind a campus building on Wednesday morning, when wind chill temperatures in Iowa City were -46C (-51F), according to local TV station KCCI.
Natural gas supplies were under threat in MI and Minnesota, where authorities asked residents to reduce their heat consumption if at all possible. By Thursday, Consumers Energy said it was "cautiously optimistic" that its requests to curb natural gas use were "having a positive effect".
Aside from the safety risks and the physical discomfort, the system's icy grip also took a heavy toll on infrastructure, halting transportation, knocking out electricity and interrupting water service.
The blast of polar air that enveloped much of the Midwest on Wednesday closed schools and businesses and strained infrastructure with some of the lowest temperatures in a generation. Several families who meant to leave for Pennsylvania stood in ticket lines at Chicago's Union Station only to be told all trains were cancelled until Friday.
In Saco, Maine, firefighters had to worry about frostbite, slippery conditions and frozen gear as they chipped away at ice and snow to move hoses.