Anna Bågenholm’s core temperature of 13.7 degrees Celsius (56.7 degrees Fahrenheit) at rescue, after approximately 80 minutes under ice, with full neurological recovery — this was considered extraordinary when it happened in 1999.

It was not considered impossible, because the medical understanding of cold’s protective effects on the brain had advanced enough that survival, with proper treatment, was theoretically possible.

Cases of even lower core temperatures — 9 degrees Celsius (approximately 48 degrees Fahrenheit) — have been reported in patients who survived with aggressive ECMO-based rewarming.

These cases have each been greeted with a combination of surprise and the expansion of the known boundaries of survival.

The pattern that has emerged from this accumulation of extreme cases is not of a line below which survival is impossible, but of a zone — a region of uncertainty in which the outcome depends on a complex interaction of core temperature, exposure duration, peripheral damage, cardiac rhythm, the speed and quality of medical response, and individual physiological factors that are not fully characterized.