the odds of this happening? A wife sees on facebook that a couple is lost, trapped in the snow, so she asks her husband to find them in a park covering 3,400 square miles, and he does. Abc's alex... See More
the odds of this happening? A wife sees on facebook that a couple is lost, trapped in the snow, so she asks her husband to find them in a park covering 3,400 square miles, and he does. Abc's alex perez with the story. Reporter: It was the wrong turn that nearly killed them. Mark and kris wathney following their gps out of yellowstone national park driving onto a snow-packed highway not know it had been closed. We never saw any road closed sign. We never saw any barricades. Then all of a sudden it went from a little bit of snow to a foot, foot and a half drifts. Reporter: Their car got stuck temperatures plummeted below zero and daylight turned into night. Snowing. No sun. Reporter: The team searched for three days on the ground and by helicopter, but no luck for six excruciating days they huddled together, putting on every piece of clothing they had, turning the car on just once every four hours and rationing the bread and water and half a jar of jelly that they had. I lost it. I spent actually most of the time we were up there I was shaking and crying. Reporter: With hope fading they decided to write letters to their families. Just letting them know that i loved them. Reporter: Never imagining it would be facebook that would save their lives. Donna who runs a ranch with her husband todd saw the local newspaper's facebook post a the missing couple and alerted them. I didn't get much sleep that night thinking about it. So I was up early trying to get my snow mobile ready so I can at least go look. Reporter: Todd set out to search for these strangers on his snow mobile. One hour later they heard the roar of the engine. Todd insists he's no hero. I was just out to help somebody. I ain't -- I don't want a title or anything. I was just out there to help. Reporter: But for mark and chris, he was the miracle they hoped for. Alex perez, abc news, chicago.
This transcript has been automatically generated and may not be 100% accurate.