Ten players from several countries competed on Sunday in London in an unusual underwater chess championship that combines intellectual sports with immersion, Reuters reported.

For a chess championship, the clothes are somewhat unusual: everyone wears swimsuits and glasses.

And at the World Underwater Chess Championship in London, chess clocks are replaced by how long participants can hold their breath.

Underwater chess is similar to regular chess, but played underwater using magnetic pieces. Each player must hold their breath during the move. After going on the air, it’s the opponent’s turn.

For four hours on Sunday, 10 players dived and strategized in the pool of the Leonardo Royal Hotel.

“I thought it would be easy, but it definitely wasn’t. Trying to see the pieces, staying there is a lot more difficult, especially if you’re playing the long game,” said player Zarein Dolab.

The winner was 33-year-old Pole Michal Mazurkiewicz, who defeated South African Alain Dekker in the final match.

“I think 60% is chess and 40% is other skills – swimming, body posture, pressure and breathing,” Mazurkiewicz said of the skills required for the sport, which combines chess with scuba diving.

The unusual championship is the brainchild of American Ethan Ilfeld, who wanted to include a physical element to make the game even more challenging.

Ilfeld, who now lives in England, started playing chess at the age of 4 and is a chess master.