It's the middle of winter in the northern hemisphere, and it might not feel like the season for jumping in the water, but more and more people are finding an icy plunge is invigorating.

William Brangham takes a look at the world of winter swimming and its potential benefits and risks.

Even though the temperatures are plunging, so are these swimmers, a brave group in the northern

Chinese city of Harbin, taking a sub-zero jump one by one into the icy water.

I felt prickling all over my body. The water here is about 10 degrees Celsius lower than it is

in my hometown, but it still made me feel blissful.

These hearty souls are part of the city's winter swimming team, which boasts hundreds of members.

They say they train throughout the year to develop the physical strength and mental fortitude needed

to dive into pools carved from ice during the city's annual winter festival.

Once these challenges are overcome, you will feel the endless joy brought by winter swimming.

Endless joy or crazy obsession. In the Czech Republic, locals there take to the cold waters of the Voltava River in early January

to mark the end of the Christmas season.

Well, I've stuck with it.

It's a bit of an addiction now.

Once you've tried it, you have to keep doing it.

In the Danish capital, Copenhagen, it's a tourist draw to brave the icy water.

It's crazy, it's really refreshing. I think it's an experience that every single person has to have once in their lifetimes.

Here in winter.

Yes, in winter, and I love it. It's really cold, but we love it.

Devotees of winter swimming are convinced these bone-chilling dips boost their health and happiness.

Back in the day, I used to get leg cramps, and ever since I started winter swimming, it has stopped.

I'm 76 years old, but I've never got a cold because I swim in winter and I feel very well.

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