She visits the Brain Science Lab at Indiana University, which studies the hard- swimming members of the US Masters: men and women who swim 3, 500-5, 000 metres three to five times a week, sometimes for decades on end.
She talks to Olympians and to open-water champions like Lynne Cox to find out why they live to swim, why swimming is man's most sensuous sport (because skin is his largest sensory organ) and why it keeps your arteries elastic, your joints supple, is good for your heart as well as your brain, and offers more than a cursory help in the fight against ageing.