High schools have begun alerting parents to a new fad.
It involves challenging friends to jump into freezing water in efforts to raise money for charity.
The phenomenon began last month as a fundraiser for a 6-month-old cancer patient in Missouri.
It's since spread through social media, but officials report an increasing number of broken bones, blown-out knees and worse.
Officials in Illinois, Minnesota and Wisconsin warn that divers can easily misjudge the water's depth or land on a submerged object.