Ernest Shackleton and his team’s 1916 Antarctic exploration became a fight for their lives when their boat, the Endurance, got trapped in ice.

The route to safety was grueling. Shackleton and two companions, Tom Crean and Frank Worsley, embarked on a journey across mountain ranges and glaciers to a whaling station in Stromness Bay. For more than a day, the men trekked across the island of South Georgia. Against all odds, they survived.

They largely credit their survival with a fourth traveler. A presence they all felt — which provided hope and direction — but was never really there.

“I know that during that long and racking march of thirty-six hours over the unnamed mountains and glaciers of South Georgia it seemed to me often that we were four, not three,” Shackleton wrote in “South,” his book about the expedition.

Frank Worsley also wrote of the bewildering experience.

“There was indeed one thing about our crossing of South Georgia, a thing which I have never been able to explain. Whenever I reviewed the incidents of that march I had the sub-conscious feeling that there were four of us, instead of three,” wrote Worsley, per the Royal Scottish Geographical Society.

Several adventurers and others who experienced life-threatening circumstances tell similar stories — that an additional person appeared and guided them to safety. This experience is what is known as “third man syndrome” or “third man factor.”

What is third man syndrome?

Third man syndrome describes the strange phenomenon experienced by explorers or others during survival situations in which a presence intervenes at a critical moment to offer encouragement, guidance and support. Those who have experienced third man syndrome report feeling a presence despite being alone.

Third man syndrome is typically experienced by individuals rather than groups. The title derives from a T.S. Eliot poem, “The Waste Land,” which references “a third who walks always beside you.”

Excerpt from “ The Waste Land” by T.S. Elliot

“Who is the third who walks always beside you?

When I count, there are only you and I together

But when I look ahead up the white road

There is always another one walking beside you

Gliding wrapt in a brown mantle, hooded

I do not know whether a man or a woman

—But who is that on the other side of you?”

Is third man syndrome real?

Writer John Geiger — author of “The Third Man Factor: Surviving the Impossible” — claims there are several scientific explanations for third man syndrome, such as biochemical reactions to misfiring brain activity, per NPR. But he believes a spiritual explanation makes more sense.

“Clearly there is a spiritual or religious explanation to this phenomenon,” Geiger told NPR. “Many skeptics and nonbelievers also had this experience and they attribute it to other explanations and there is certainly some very interesting science behind this.”

“If we understand that the Third Man Factor is a part of us, the way adrenaline is ... then we can start to access it more easily,” Geiger added, per NPR. “It’s not a hallucination in the sense that hallucinations are disordering. This is a very helpful and orderly guide.”

No one really knows what causes third man syndrome. Many people claim it is a hallucination or coping mechanism triggered by extreme stress while others believe it is help given by a guardian angel, per CNN.

“It’s an astonishing capacity if you think about,” said Geiger. “And it sort of hints at this idea that as human beings we are never truly alone, that we have this ability to call upon this resource when we most need it in our lives.”

Accounts of third man syndrome

Geiger spent five years gathering stories from individuals who have experienced third man syndrome. He chronicles the mysterious experiences in his book “The Third Man Factor: Surviving the Impossible.” Here are a couple accounts given in the book.

Ron DiFrancesco was on the 84th floor of the South Tower at the World Trade Center on September 11, 2001. When the second plane crashed into the building, he made several attempts to escape but failed to find a safe exit. At the brink of giving up, DiFrancesco heard an unfamiliar voice.

“‘Someone told me to get up.’ Someone, he said, ‘called me.’ The voice which was male, but did not belong to one of the people in the stairwell was insistent: ‘Get up!’ It addressed DiFrancesco by his first name, and gave him encouragement: ‘It was, ‘Hey! You can do this.’ But it was more than a voice; there was also a vivid sense of a physical presence,” Geiger wrote in “The Third Man Factor: Surviving the Impossible.”

“He had the sensation that ‘somebody lifted me up.’ He felt that he was being guided: ‘I was led to the stairs. I don’t think something grabbed my hand, but I was definitely led.’”

With help from what DiFrancesco describes as “an angel” he made it out of the South Tower alive. He was only one of four people to escape from above the 81st floor.

While ice climbing in the Canadian Rockies near Lake Louise, Alberta, James Sevigny and Richard Whitmire were overcome by an avalanche. It carried them nearly 2,000 feet. While Sevigny regain consciousness, he was severely injured — and he discovered that Whitmire was dead.

His back was broken in two places, he had a fractured arm, cracked ribs, severed nerves, torn ligaments in his knees, broken teeth and internal bleeding. He laid down to die.

“He then felt a sudden, strange sensation of an invisible being very close at hand. ‘It was something I couldn’t see but it was a physical presence.’ The presence communicated mentally, and its message was clear: ‘You can’t give up, you have to try,’” Geiger wrote in “The Third Man Factor: Surviving the Impossible.”

“The presence urged Sevigny to get up. It dispensed practical advice. It told him, for example, to follow the blood dripping from the tip of his nose as if it were an arrow pointing the way. As he walked, he kept breaking through the crust of the deep snow, and was almost unable to pull his feet back up because of his injuries. Part of the time he crawled. The presence, which stood behind his right shoulder, implored him to continue even when the struggle to survive seemed untenable.”

When Sevigny finally made it back to base camp, there were people there to aid him. When the presence left, he was overcome with loneliness. He understood it was more than luck that led to his miraculous survival.