The couple, from Idaho Falls, Idaho, were rescued on Monday NZT following a weekend adventure-turned-catastrophe which left the two fighting simply to stay alive.
The Christiansens set out for what they thought would be a day-long excursion to the Darby Canyon Ice Cave to celebrate Spencer's 31st birthday.
Spencer said he and Jessica, 24, had been researching the cave and, based on the information available to them, they thought they had a handle on it. When they reached the entrance, he said, they put on all their clothes, turned on their flashlights and made their way inside the tall, narrow cave.
"It was beautiful," Spencer said, adding that it was so cold in the cave that there was ice on the walls, which sparkled as the light from the couple's headlamps hit them.
But, the couple said, after hours and hours of climbing over rocks, squeezing through tight passages and wading through frigid water, they realised there was no way out.
Then the couple, who were shivering cold and wet, spotted a rope dangling down from the ceiling, but by that time, their arms and legs were cramping and their hands and feet were numb.
"We thought that was going to be the only way out," Spencer said about the rope, which he said was intertwined in a small waterfall. "All we could imagine to get out was climbing that rope in the waterfall."
"But we didn't have enough strength or warmth to climb it," Jessica interjected.
They were exhausted.
And they were hungry.
Spencer said the two rested, burned their garbage for warmth and ate to try to regain their strength - then they tried it again.
And again, they failed.
"The more we tried to climb it, the colder and wetter we got," Jessica explained.
As a precaution, the couple said that they had instructed Jessica's mother to call a search-and-rescue crew if they did not return, so they hoped help was on the way.
As they waited, they backtracked to try to find another way out, but couldn't see one. They exercised - doing jumping jacks, push-ups and squats to keep themselves awake to avoid losing more body heat.
Then they huddled together and made another fire, this time, using whatever they could find - Spencer's knee brace, a wad of Jessica's hair, a backpack, hats, a pair of gloves. "We got warmer," Spencer said, "but then the fire went out."
And the rescuers still hadn't come for them.
"It got to a point where we understood that we were going to have to climb out," Spencer said, adding that they devised a plan to use a pulley system and give it one last try.
This time, the couple said, it seemed to be working - Jessica had climbed up the rope into a hole and although the pair were completely soaked, they were amazed they were doing it. But as Jessica grabbed onto some rock to pull herself up, it broke away and she fell.
"I was able to catch her," Spencer said, "but she bounced off some rocks."
Then Spencer took the lead and, eventually, the couple made it to the top of the rope - but the exit was closed off, he said.
"That was our last-ditch effort to survive," he said. "We were completely shocked. We were completely terrified. We really started wondering whether we were going to live or not."
The couple said they climbed back down.
They were there the entire day. We had no idea
And that's when they heard voices.
"We realised there were people down there," Spencer said, adding that they started screaming and the rescuers found them.
Teton County Undersheriff Matt Carr told the Jackson Hole News & Guide that Jessica's mother did indeed call for help.
Carr said a crew from Teton County Search and Rescue made the way to the cave and searched where other explorers had been trapped in the past, but did not see them.
"We had teams going through, and they could smell smoke. They eventually noticed a fixed rope coming down near a waterfall," he told the newspaper, adding that it took a rescuer 45 minutes to climb the rope to reach them.
When rescuers found them, Carr said, they were "stuck and unable to move." He said that "they had gotten soaking wet in this waterfall."
But that wasn't the end of the Christiansens' long and treacherous journey. The couple, who had hyperthermia, said they still had to climb out with help from the search-and-rescue crew. But the worst was certainly over. When they got out, they said, both of their families - and their 1-year-old daughter - were there waiting for them.
"They were there the entire day," Spencer said of their families. "We had no idea."
The couple said today that the ordeal put their lives into perspective.
"The things that seemed most important before don't even matter now," Jessica said. "Things like bills or work stress. It makes me want to work less and have more family time because you don't know when will be your last day with them."
In a Facebook post after the incident, Spencer thanked family members and friends who had wished him a happy birthday - a birthday unlike any other he has had.
"This year was extremely different from any other," he wrote. "I am completely dumbfounded and honored by all the love and effort that went into this last weekend by my family . . . I will never forget the search and rescue team along with my family singing happy birthday, once we were finally safe . . . You all are amazing . . . Thank you for being who you are."
