The motivating factor? All of the proceeds go to a great cause.
Call it the "polar vortex plunge." That's what it felt like to everyone brave enough to go in.
The icy waters of Lake Michigan - so icy that chunks had to be chipped away before the plungers arrived - was a cool 32 degrees.
"Just go in and have fun, just go," said Abbie Elliot, participant.
Thousands just went, and quickly went out, of the water for the Special Olympics Chicago.
"I think it's on people's bucket lists. I think this is a crazy thing you can be able to tell your friends you've done, and I also think people really care about Special Olympics. And being born in Chicago, we have a lot of civic pride to the charity in a lot of ways," said Jen Kramer, president, Special Olympics Chicago.
An event, 14 years old, powered by hearty souls, off all ages, and all for folks like Keith Tyler, a Special Olympian.
"i think it's a wonderful experience, everyone's gonna get in the water, have a good time, even though it's gonna be freezing cold today. Everyone is gonna have a wonderful experience," Tyler said.
Chicago Mayor Rahm Emanuel also made the bold plunge into the water, fulfilling a promise he made to school children that read 2 million books.
It was a fun, albeit bitterly cold, event. One that the Special Olympics hopes brings in their most funds yet. Every dollar raised Sunday goes directly to Special Olympics.
Every year, hundreds of people take the plunge to raise money for Chicago's Special Olympics.
"Getting that site prepped for this takes a week," Michael Kelly, general superintendent of Chicago Park District, said. "Driving tent stakes doesn't happen overnight because the ground is frozen. So it's a big deal."
Frozen ground, and frozen water.
"You can't thank enough the volunteers and park district personnel who were out there removing 3 feet of ice this week and constantly breaking it up to get ready for Sunday," Kelly said.
"I made a bet to the children to the City of Chicago and I took a whooping," Emanuel said. The mayor promised to participate if Chicago children read two million books. He and former Chicago Bear Israel Idonije spoke about the event on Windy City LIVE.
"And Mayor, I know you wanted a recount of books read. We keep pretty fastidious records at the library. Kids read not only 2 million books, they read 2.1 million books," Liz McChesney, Chicago Public Library, said.
Special Olympics, which was founded in Chicago in 1969, supports programs year round.
"We are at 3,000 plungers and are looking to breaking our fundraising records of $1 million," Kramer said.
POLAR PLUNGE: The effect on the body
"The cold temperatures when you dive into water immediately can be somewhat hazardous," said Dr. Dipul Patadia, an emergency medicine physician at Advocate Good Samaritan Hospital.
Sudden immersion in very cold water can cause external blood vessels to constrict. Blood pressure goes up, and the heart has to work harder than normal to pump blood through the body.
Generally speaking, people who are healthy should be fine, doctors say. Elderly people, children and anyone with major medical problems should consult with their doctor before taking part in the Polar Plunge.
It's also not a good idea to drink alcohol before jumping into icy water, said Dr. Sari Hart, emergency physician with NorthShore University HealthSystem, though one could argue that might make it more bearable.
"People who have alcohol in their system are less likely to accurately sense cold and less likely to respond appropriately to cold exposure," Hart said. "They might not shiver, they might not seek warm dry clothing right away, and they would be at increased risk for hypothermia."
At these temperatures, people shouldn't stay in Lake Michigan longer than five minutes. More than 30 minutes can cause a person to die of cardiac arrest, Patadia said.
The Centers for Disease Control and Prevention also recommends the following precautions for cold water immersion: wear proper clothing, avoid cotton, instead go for wool and synthetics; use a personal flotation device and have someone there who can retrieve you from the water.
The Sun-Times Media Wire contributed to this report.