Prolonged stable hypothermia during a 10-hour cold open-water marathon swim

Marathon open-water swimming presents extreme thermophysiological challenges, particularly in cold environments. This case report describes continuous core temperature data from a solo crossing of New Zealand's Foveaux Strait, an infamously cold (13°C -14°C), turbulent open-water swim. A 52-year-old male swimmer (body mass index, 27.9 kg m^-2; body fat, 18%) completed the 37 km swim in 9 h 52 min under standard marathon swimming rules (no wetsuit). Core temperature (measured via an ingestible thermometer pill) decreased rapidly after immersion, falling from 37.9°C to <35.0°C within 50 min. The swimmer reached a nadir of 33.88°C at 2 h 42 min and remained hypothermic for more than half of the swim (total, 369 min). Despite this, he displayed no overt cognitive or motor impairment, completed the swim unaided and did not experience an after-drop post-immersion. This case highlights the remarkable thermoregulatory tolerance of a cold-adapted endurance swimmer.