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. Highlights - What is the main observation in this case? A cold-adapted marathon swimmer tolerated prolonged moderate hypothermia (core temperature <35°C for >6 h; nadir 33.88°C) during an open-water swim in 13-14°C water. - What insights does it reveal? This case demonstrates that in a trained and cold-adapted marathon swimmer, prolonged immersion in cold water can be tolerated at core temperatures traditionally classified as clinically hypothermic, without functional compromise or adverse outcomes.
© Copyright 2026 Experimental Physiology. The Physiological Society. All rights reserved.