CHEYENNE, Wyo. (AP) – An acclaimed poet who had been working on a book about volcanoes disappeared five days ago during a scouting expedition on a tropical island in southern Japan.

Craig Arnold, 41, is an assistant professor of English at the University of Wyoming. He went missing Monday during a visit to a volcano on the island of Kuchinoerabu-jima in the northern Ryukyu Islands of southern Japan.

Japanese law only requires authorities to look for missing people for three days, but University of Wyoming officials say the search has been extended through Sunday.

Arnold went for a hike up the volcano around mid-afternoon Monday, shortly after arriving at the island by ferry and checking in at an inn, according to his brother, Chris Arnold, of Brooklyn, N.Y.

When Arnold hadn’t returned by 8 p.m., the inn staff went looking for him. They reported him missing at 9 p.m., and a formal search began that night.

Forty people, dogs and a helicopter joined the following day’s search. Police reported finding Arnold’s tracks on a trail up the volcano, but they couldn’t find any tracks coming down.

The island is about 7 miles long by 3 miles wide and dominated by an 1,800-foot volcano that last erupted in 1980.

Dense vegetation covers much of the island, but the area near the caldera, the volcanic crater, is bare. Japanese authorities speculate that after emerging at the top, Arnold may have had difficulty finding the trail again to get back down the volcano, Chris Arnold said.

The thick terrain makes helicopter searches virtually useless, said Chris Arnold’s wife, Augusta Palmer.

“So they’re really having to do it by lines of people walking across areas to make sure they’re covered,” she said.

Arnold has published two award-winning collections of poetry: “Shells” in 1999, and “Made Flesh,” which came out last year. He’s known for his searching, passionate, humorous verse, such as “Hymn to Persephone,” which begins, “Help me remember this/how once the dead were locked/out of the ground/and wandered sleepless and sun-blinded.”

Chris Arnold said his brother lived in Japan for four years as an army brat and has since been a world traveler who has explored many volcanoes for his book.

“He has a lot of experience with visiting volcanoes, and he’s very knowledgeable about taking precautions and knowing about the terrain, about the conditions, possible ways he could get into trouble and to plan for those,” Chris Arnold said.