At 22 km (14 miles) in width, New Zealand’s Cook Strait is hardly the wides body of water in the world. Still, it is a formidable challenge to undertake that crossing, which has plenty of turbulent water, strong currents, and cold temperatures that can test the endurance or any athlete. But for world-champion free diver William Trubridge, just swimming across the strait wasn’t challenging enough. He had to do it underwater instead, coming up to the surface just to collect more air. In this video, we join him on this venture, in what turns out to be every bit as difficult as he had imagined.