In August 1914, British explorer Ernest Shackleton and a crew of 27 sailed toward the South Pole on the ship Endurance in an effort to become the first party ever to cross the Antarctic.

Five months into the journey heavy pack ice closed in, slowing the Endurance's progress, then halted it completely. The men were forced to abandon ship. Camped for almost a year on the frozen sea with their sled dogs and lifeboats, they could only watch as the ice pressed tighter and tighter against their ship, finally crushing it to splinters.

Shackleton and his men dragged and paddled those lifeboats across hundreds of miles of polar wasteland, trying against all odds to reach civilization before the elements did them in. With the spring thaw, the group at last reached Elephant Island, a wind-whipped rock in the South Atlantic Ocean. The crew built makeshift shelters there.

Shackleton and five others set off in one of the lifeboats for a whaling station on South Georgia Island, just above the Antarctic Circle. After a treacherous and near-fatal crossing, the men landed safely. Leaving three men behind with the boat, Shackleton and two other crew members, Captain Frank Worsley and Second Officer Thomas Crean, set out for the whaling station on the island's far side.

It was on this grueling, final leg of the ordeal that something very strange happened. "During that long and racking march of thirty-six hours over the unnamed mountains and glaciers of South Georgia," Shackleton later wrote, "it seemed to me often that we were four, not three."

Thinking that this mysterious fourth man accompanying them must have been a figment of his imagination—brought on no doubt by exhaustion and the strain that the months on the ice had placed on him—Shackelton said nothing about it to his companions.

Finally, they crossed the island and made it to the whaling station and safety. It was then that Frank Worsley confided something. "Boss, I had a curious feeling on the march that there was another person with us."

Thomas Crean then spoke up. He too had sensed a presence. Another person—invisible but felt by all three—had been with them on the ice.

What was the explanation? "One feels," Shackleton wrote, "the dearth of human words, the roughness of mortal speech in trying to describe things intangible, but a record of our journeys would be incomplete without a reference to a subject very near to our hearts."

In 1927 Charles Lindbergh underwent a similar experience during his famous air-crossing of the Atlantic. Some 10 hours into the journey, as the Spirit of St. Louis wended its way through cloud canyons high above the ocean, the aviator found himself struggling to keep focused on his instrument panel. To drift off to sleep for even a few seconds would have meant disaster.

Growing ever groggier, Lindbergh began to get an unshakable sensation that someone was there in the plane with him. "While I'm staring at the instruments," he wrote, "the fuselage behind me becomes filled with ghostly presences—vaguely outlined forms, transparent, moving, riding weightless with me in the plane."

Lindbergh felt no fear at the appearance of these "friendly, vaporlike shapes," who "speak with human voices." In fact, he sensed they were there to help.

"First one and then another presses forward to my shoulder to speak above the engine's noise, and then draws back among the group behind. They're neither intruders nor strangers. It's more like a gathering of family and friends after years of separation."

In 1933, mountain climber Francis Sydney was high on Mount Everest when he felt himself joined by a "strong and friendly" invisible companion. "In its company," Sydney wrote, "I could not feel lonely, neither could I come to any harm. It was always there to sustain me on my solitary climb up the snow-covered slabs. Now, as I halted and extracted some mint cake from my pocket, it was so near and so strong that instinctively I divided the mint into two halves and turned around with one half in my hand to offer it to my 'companion.'"

Theodora Ward, who includes this story in her book “Men & Angels,” tells of a Scottish couple who were trailed by an "unseen presence" while crossing the Greenland icecap. Ward makes the point that in the overwhelming majority of these encounters, the presence is specifically described as being friendly.

Perhaps not surprisingly, such magical experiences continued to be reported when astronauts began to explore outer space.

In 1971, Apollo 14 astronaut Edgar Mitchell, while reentering the earth's atmosphere after a moonwalk, experienced what he described as "an overwhelming sense of universal connectedness. There was a sense that our presence as space travelers, and the experience of the universe itself, was not accidental....The thought was so large it seemed at the time inexpressible, and to a large degree still is."