A woman in Tampere, Finland, takes part in the "avanto" ice-swimming ritual in January. Finnish photographer Markku Lahdesmaki documented the phenomenon in his hometown.
Lahdesmaki went with his camera to nearby Rauhaniemi Lake, which was "teeming with swimmers" despite below-zero temperatures.
The "avanto" ritual, which literally translates to "hole in the ice," is combined with another Finnish pastime: the sauna.
Swimmers brave the icy water either before or after a session in the dry heat of the sauna.
Having arrived at the lake "dressed like a North Pole explorer," Lahdesmaki said he eventually stripped off and joined the swimmers.
Lahdesmaki's portraits show swimmers facing the elements in nothing but a swimsuit.
When you're exposing your body to sub-zero temperatures, it's probably a good idea to sling on a hat.
Winter swimming is a popular activity around the world, most commonly around New Year's Day and the Christian festival of Epiphany.
Ladesmaki said that after taking the plunge, he "ended up swimming six times." "I was very moved by the experience and the people I met," he said.