As temperatures plunged deep below zero in several states, schools and state government offices shuttered, thousands of flights were canceled and at least nine people died in unbearably cold and snowy conditions.

Hundreds of public schools and universities from North Dakota to Pennsylvania canceled classes as students huddled to stay warm amid the polar vortex.

In addition to schools, major attractions, such as zoos and museums were closed. So, too, were courthouses.

The show couldn’t go on in Chicago, where “Disney on Ice” and “Hamilton” were canceled. Teacher Andrew Conneen quipped on Twitter, “Brrrrrrr kills Hamilton again.”

United States Postal Service carriers may keep going in rain, heat and gloom of the night — but they called it quits during the arctic blast — taking the rare step of suspending mail delivery across the region.

Meanwhile, more than 1,600 flights were canceled out of Chicago’s airports Wednesday, including 1,300-plus just at O’Hare — where a morning temp minus 23 degrees set the record for the coldest since Jan. 30, 1966.

It was also the coldest Jan. 30 since 1966 in Mansfield, Ohio, and South Bend and Fort Wayne Ind. In Indiana, where it was at least minus 11 degrees, 4,000 people lost power.

The deep freeze has also caused some rare weather conditions.

The Chicago River froze and was emanating plumes of water vapor nicknamed “sea smoke.” Loud booms were heard across the Windy City in what may have been frost quakes. Those happen when a sudden drop in temperature causes water in the ground to freeze and expand — creating a pressure buildup that makes a boom noise.

Officials said that at least nine people had died in the cold — including a former City Council member from Ecorse, Mich. and a 70-year-old man in Detroit who were found frozen.

Two more men froze to death Tuesday, including an 82-year-old man who was found suffering from hypothermia outside his home in Pekin, Ill., where the high was minus 11 degrees.

A 55-year-old man who’d been shoveling snow was also found frozen to death in a detached garage near his home in Milwaukee County, Wis.

With Post wires