Police have told the family Arnold's footprints were found ascending, but not descending, the mountain path. The one-mile diameter caldera -- or summit area -- was extensively searched and nothing was found.
Authorities suspect that Arnold may have wandered off the trail into dense forest that would have darkened at nightfall. The inn owners called the fire brigade at 9 p.m. when he failed to return, searching until midnight.
Rescue teams have used infrared techniques to look for body warmth and searched the holes and depressions where Arnold could have fallen. Locals with knowledge of the landscape are helping.
The Japanese called off the search after three days, but U.S. officials pushed to have the search extended another three days. They have told the family there is no danger of exposure with spring-like temperatures and that fresh water and food sources are available.
Arnold's mysterious disappearance poses as many contradictions as the last entry in his blog, "Volcano Pilgrim: Five Months as a Wandering Poet."
There, Arnold wrote about the plant angelica, a member of the apiaceae family, which includes both the sweet herb cilantro and deadly hemlock, which he noted killed Socrates.
Japanese rescue workers think Arnold may be hurt.
"Naturally, I don't want to believe that," said his brother. "But he may be less mobile, which would explain why he is in the woods."
Because Arnold has traveled so widely, he has a safety protocol with his partner of six years, Rebecca Lindenberg: If she doesn't hear from him in 48 hours, get help.
In this case, it was the Japanese authorities who contacted her.
"I was in complete and total shock, almost nonfunctional with worry and anxiety," said Lindenberg, 31, who is a Ph.D. student at the University of Utah and agreed to be the person to "stay on the ground" in the United States.
"We take it a day at a time," she told ABCNews.com. "It's quite mysterious, and nobody has been able to reconstruct a narrative for what has happened to Craig.
"He's not an enormous risk taker," she said. "He knows he's not a volcanologist, he's a writer."
As Chris Arnold heads off to Japan, Lindenberg is finding ways to "honor Craig as well as keeping him alive," creating a Facebook page to update friends and university colleagues.
"What has also been amazing and unfathomable and unexpected is the outpouring of concern from the literary community and others that just encircled us," she said.
The circumstances under which Arnold disappeared reflect the themes of his poetry -- "the vivid tactile and sensual nature of experience," according to Lindenberg.
"He is a poet who is, in the words of one of his own poems, 'full out to the skin,'" she said.
As she awaits news about his fate, she thinks of the last line in Arnold's poem, "The Singer," referring to a bird song:
And what they sing so lovely and so meaningless
may urge itself upon you with the ache
of something just beyond the point of being remembered
the trace of a brave thought in the face of sadness.
Arnold's sister-in-law, Augusta Palmer of Brooklyn, N.Y., is also reminded of his poem, "Couple From Hell," hoping that he, too, is only in hiding and will soon re-emerge.
His philosphy, Palmer told ABCNews.com, is "poets should go where ordinary people can't or won't go to tell what experience is like."