In 1933, British explorer Frank Smyth almost became the first person to reach the summit of Mount Everest. The journey to the top of the mountain was arduous and nearly disastrous; his entire hiking party had fallen back, unable to make it through the sweeping wind, snow, ice and low oxygen. Smyth continued, but never made it to the top — he missed it by 1,000 feet.

Later, writing in his diary, Smyth described something that scientists commonly refer to as the "Third Man Factor." He recounted how at one point on the ascent, he reached into his pocket, pulled out a slab of Kendal mint cake, broke it in half and turned around to give the other half to a companion. But there was no one there: "All the time that I was climbing alone, I had a strong feeling that I was accompanied by a second person. The feeling was so strong that it completely eliminated all loneliness I might otherwise have felt."

Writer John Geiger chronicles the phenomenon of the phantom companion in his new book, The Third Man Factor: Surviving the Impossible. As Geiger explains, the Third Man is an unseen being that intervenes at a critical moment — when people are in great stress or in a life-and-death struggle — to give comfort, aid or support.

"Clearly there is a spiritual or religious explanation to this phenomenon," Geiger tells Guy Raz. But he also says there is strong science behind the Third Man: "Many skeptics and non-believers also had this experience and they attribute it to other explanations and there is certainly some very interesting science behind this."

Geiger spent five years tracking down the stories of people who've experienced the Third Man phenomenon. He opens his book with the story of Ron DiFrancesco, a worker at the World Trade Center on 9/11.

DiFrancesco was on the 84th floor of the South Tower when the second plane struck. He tried to make his way down the stairwell, but was forced to lie down to avoid a raging fire and thick smoke. It was then that he recalls feeling something grab his hand and lead him out. DiFrancesco was the last person to leave the South Tower before it collapsed.

Geiger says that the scientific explanations behind Third Man range from bio-chemical reactions to misfiring brain activity.

"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," he explains. "It's not a hallucination in the sense that hallucinations are disordering. This is a very helpful and orderly guide."

One of the most famous instances of the phenomenon took place during Ernest Shackleton's Antarctic expedition in 1916. The team's boat was trapped in ice and they were forced to make a grueling journey across mountain ranges and glaciers to a whaling station in Stromness Bay. Shackleton later wrote: "I know that during that long and racking march of thirty-six hours over the unnamed mountains and glaciers, it seemed to me often that we were four, not three."

Later, the poet T.S. Eliot read Shackleton's account of a mysterious "fourth" man and took some poetic license with the idea, including it in his famous poem, The Waste Land. He turned Shackleton's fourth into a third — and this is where the phenomenon gets its name:

"It's an astonishing capacity if you think about," Geiger says. "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."

Copyright 2023 NPR. To see more, visit https://www.npr.org.