Burleigh McCoy

Almost every year since 2011, a huge lake in Madison, Wisconsin, has frozen over enough to support an entire festival.

It's called Frozen Assets, and that's where I headed to report this story.

Ooh.

So on and near the ice of Lake Mendota, there is ice hockey, curling, ice skating.

People are flying these giant, colorful kites on the ice.

A skydiver is landing on the ice right now.

That's incredible.

And they did this warm-up for a 5K where more than a thousand people jump up and down on the ice to make it shake.

So this is James Tai, the executive director and founder of Clean Lakes Alliance, which puts on the festival.

But Emily, this lake, which freezes every year, this year with ice thick enough that it could have held an entire fleet of trucks, might one day not freeze at all.

And thousands of others may stop too, like soon.

Plus, I go out with Madison's lake rescue team to see how they train for emergencies when someone falls through.

Yeah, so I double-checked this with Hilary Dugan at the festival.

She's a scientist who studies inland lakes, and she says it's safe for a person to walk on ice when there is four inches of black ice.

So black ice, which is called that because it's clear and appears black, is strongest.

But it's not the only type of ice.

Because of all the air, white ice scatters more light, isn't clear, and it looks white.

It's less strong, and so it's less safe.

And unlike black ice, it's tricky to say how much white ice is safe to walk on because the amount of air in it can be variable, and it's often mixed with black ice.

Absolutely.