The first underground swimming relay was held in Poland at the Historic Silver Mine in Tarnowskie Gory. It's on the UNESCO World Heritage List. The brave participants seem to love it!
"People who swim open water know what it means," said one female swimmer. "This water is soft, you can feel it with the whole body, the experience is almost spiritual, it's fantastic."
Three hundred winter swimmers, known as 'Walruses' took part in the event which saw them swim through a 600 metre long rock corridor, 30 metres underground.
Piotr Biankowski says his experience of swimming across the dangerous English Channel helped.
"It's amazing, I would recommend it to anyone who would like to cross the English Channel, I would recommend trying to swim in this darkness, I also started crossing the channel around midnight. Such training will be useful to everyone."
After the exhilarating encounter with water of around six degrees celsius, and an air temperature of around 10 degrees, the swimmers could put their layers back on to warm up.