You may have seen a mermaid show at an aquarium. What sets this sport apart from those performances is that this kind of mermaid swimming is combined with freediving: swimming into very deep ocean waters for extended periods of time--without dying.
Freediving looks beautiful, but it's also dangerous. Highly skilled, trained swimmers without breathing equipment reach depths of up to 253 meters. Ian Donald, a freediving instructor in Cornwall, UK, teaches people how to do this while wearing mermaid fins over their legs. Susan Greenwood of The Guardian describes the training:
The key to being happy under water for long periods is, I discovered – after panicking – not to panic, to keep the heart rate low by “breathing up” or belly breathing for two minutes before submersion, and to know that humans take a breath long before we need to. It’s the rule of thirds: for the first third of the time underwater, people are happy; the second brings the urge to breathe and it’s at this point most of us surface. But, resisting this urge means entering the third phase – the spleen will release more red blood cells, the diaphragm will stop juddering and this is followed by the discovery that the body has more oxygen than imagined. […]
Just as things were getting serious, Lissie lined up the mermaids’ tails, handed me a seashell necklace and informed me my name was now Sue-Sea. And this is when things got gnarly. Professional mermaids wear tails that can weigh up to 40kg and cost over £2,000. Have you tried swimming with a small child holding onto your legs and making it look effortless?
-via Nag on the Lake
The Extreme Sport of Mermaid Swimming