More recently, controversy was stirred in Russia when three cosmonauts claimed to see the faces of seven angels while orbiting earth.

The message of these stories seems clear. On mountainsides and snowfields, high in the air or in the far reaches of space, the walls that can keep us closed off from spiritual domains can, at moments, fall away.

When they do, people can find themselves on what Lindbergh called "the borderline of life and a greater realm beyond." A place where "death no longer seems the final end it used to be, but rather the entrance to a new and free existence." Seeing and sensing with a newfound openness, these explorers of the physical world became explorers of the world of spirit too.

Once they return things may change. Back in ordinary life, the temptation can be strong to dismiss these magical encounters.

When he finally landed in Paris at the end of his 33-hour flight, Lindbergh was for a moment the most famous person in the world. Millions hung on his words, hungry for every last detail of his crossing. Yet he waited over 25 years before telling anyone of those "strange passengers" who had kept him company in the sky.

Likewise, Shackleton needed first to hear from his two companions before he dared tell of what he had felt out there on the ice. As explorers, as conquerors of the unknown in an age when science and reason were supposed to be sweeping the last vestiges of superstition from the world, they weren't supposed to take such angelic visitations seriously.

Ultimately, though, they did, and the world is better for hearing their accounts. Now, just as in the days when three shining strangers appeared to Abraham beneath the oak at Mamre, angels wait to guide and help us, unseen until the moment when we are ready to become aware of their presence. Of all the treasures of the universe that the world's explorers returned home with, what could be more valuable than that?