The last glacial period on earth began to fade around 11,000 years ago.
The kilometers-thick ice-sheets that covered much of the earth started to recede, and the earth entered into the Holocene, the current geological epoch.
But the once mighty glaciers did not disappear entirely, and the extremes of our earth - the north and south poles - are the remnants of the last ice age.
These frozen tundras easily reach temperatures as low as -40°C, and sometimes dip far below that.
Darkness envelops them for entire months at a time.
They are places that seem wholly inhospitable to any living creature.
Temperatures are so low that hypothermia can quickly set in, and ice itself can form inside the body - its crystals shredding tissue, ripping cells apart.
But for millions of years, evolution has forged remarkable adaptations across the animal kingdom to ward off the icy grip of death.
It's the reason Arctic animals have specialized heat generating tissue, and the reason polar bears are invisible in infrared vision.
It's what has shaped our circulatory system, and why so many traits appear in cold weather animals that appear nowhere else in the evolutionary tree.
