In the harsh winter of 1950, American troops were surrounded in South Korea—when the Communist Chinese Army trapped UN soldiers and a peacekeeping force of Marines. Assisted by air support, our badly outnumbered troops battled frostbite, hunger, and exhaustion—managing to carry their wounded and to fight their way 90 miles to the sea. Today, Marines have not forgotten Korea, and they now prepare for harsh, cold-weather conditions in northern California’s Sierra Nevada mountains during winter.

We see them rope up and leap into an ice-cold water accessed through cuts in ice-covered lakes—and we learn that the exercise is about more than withstanding the cold. A Marine on camera says, “It’s being the sort of person that can deal with anything without fear. That’s the sort of person that you can send anywhere.”

The mountains provide rugged terrain, where Marines hike, rock climb, and rappel, create one-rope bridges, zip-line, and manage injuries in the field—no matter the season or the temperature. It’s some of the most dangerous training in a Marine’s roster.