(CNN) — Whether they're sweating in the sauna or plunging into an ice-cold lake, it seems that the Finns love extremes.

And with winter just around the corner, there's no better time to indulge in the national pastime the Finns call "avanto."

The English translation for that? "Hole in the ice."

Photographer Markku Lahdesmaki documented the ice-swimming phenomenon during a trip to his hometown of Tampere, in southern Finland, earlier this year.

His portraits of hardy swimmers facing the elements in nothing but a bikini or a pair of trunks send a shiver down the spine.

'Finland is not most places'

"In most places, when you went to a lake in January, you wouldn't expect to find people swimming," Lahdesmaki tells CNN. "But Finland is not most places."

Photographer Markku Lahdesmaki Markku Lahdesmaki

He found Rauhaniemi Lake "teeming with swimmers" despite below-zero temperatures.

The avanto ritual is combined with that other Finnish staple, the sauna.

Temperatures in these traditional bath houses range between 60 and 100 degrees Celsius (140-212 degrees Fahrenheit).

Swimmers at Rauhaniemi enjoy a blast of dry heat either before or after their bout of ice swimming.

'Spiritual, breathtaking, invigorating'

Lahdesmaki arrived at the lake dressed in his thickest winter clothing.

"I wondered how the swimmers would react to my embedding myself in their avanto moments looking like a North Pole explorer.

"How could I make them comfortable? And then it hit me ... I need to join them."

The photographer underwent his baptism of ice and "ended up swimming six times."

He said, "It was an amazing experience: spiritual, breathtaking, invigorating and, of course, cold.