Freezing to death is rarely the quiet drift into sleep shown in films. This episode follows the body as its core temperature falls below the normal range of 36.5 to 37.5 degrees Celsius. Below 35 degrees, mild hypothermia brings violent shivering, a racing heart and cold-induced diuresis, as blood pulled from the limbs makes the body sense too much fluid in its core. Below 32 degrees shivering stops because cold muscles can no longer burn their fuel, and amnesia and slurred speech set in. The episode also explains why alcohol, despite the image of the brandy-carrying St. Bernard, opens blood vessels in the skin and speeds the loss of core heat.

Below 28 degrees come paradoxical undressing, seen in 20 to 50 percent of hypothermia deaths, and terminal burrowing, also called hide-and-die syndrome. Water draws heat away about 25 times faster than air, and many cold-water deaths come from cold shock and cold incapacitation within minutes. The episode covers the rule that no one is dead until warm and dead, a Polish toddler named Adam who survived a core temperature of 12.7 degrees, the cold that ravaged the armies of Hannibal and Napoleon, and how surgeons now cool patients to as low as 10 degrees and stop the heart during deep hypothermic circulatory arrest.

• Two theories of paradoxical undressing: a misfiring hypothalamus and exhausted blood vessel muscles
• Why undressed victims found outdoors are sometimes mistaken for assault victims
• The Titanic’s water at minus two degrees Celsius and the involuntary gasp reflex
• The Coast Guard’s 50-50-50 rule on life jackets in 50 degree Fahrenheit water
• Rewarming with ECMO, and tardigrades that use the sugar trehalose to survive freezing