FAIRBANKS (AP) -- A few miles north of Fairbanks stands what looks like a shack pushed back against a hill. Behind its front door dwells a distant past when saber-toothed tigers, mastodons and even camels roamed Alaska's Interior.
The shack door opens into a 35-year-old tunnel leading into a thick layer of permafrost underlying the Goldstream Valley hillside. Jutting out from the tunnel's walls are animal bones dating back thousands of years, and ancient ponds are shown buried in their entirety and frozen solid.
The 360-foot main tunnel was bored by the Army Corps of Engineers in the mid-1960s to help the military figure out how to build roads and buildings on permafrost.
The federal Bureau of Mines later excavated a 200-foot-long offshoot to test techniques for Interior Alaska gold prospectors.
In the tunnel, miners could see underground geologic formations, said Earl Beistline, now retired from a long career as a mining engineer in Fairbanks.
''Their main objective was to thaw the ground, but it helped if they could visualize exactly what they were running into,'' he said.
The tunnel system is owned by the Army Corps' Cold Regions Research and Engineering Laboratory, based in Hanover, N.H., and is mostly used by scientists.
Researchers have ventured in over the years for a variety of projects, among them assessing global climate change, gathering DNA from extinct species and collecting samples of microscopic creatures frozen into the ice.
Richard Boone, a soil ecologist at the University of Alaska Fairbanks, has yet to do any science in the tunnels, but he makes regular field trips with his biology students.
''They love it -- they're awed by it,'' Boone said. ''They get to walk back through time 40,000 years.''
Permafrost is defined as soil that has been frozen for at least two years. Ground ice is usually present, though not always. Permafrost underlies about 85 percent of Alaska, including parts of the Fairbanks area, to depths of hundreds of feet.
A thin layer of earth atop the permafrost protects it from summer's thawing warmth. When that layer is scraped off or otherwise disturbed, the underlying permafrost melts into mud.
Throughout Alaska are found residuals of construction projects where attention was not paid to permafrost's fragile nature -- buildings listing drunkenly and water-filled trenches that briefly were roads.
The section of permafrost where the tunnel was drilled is thought by scientists to be a remnant of a prehistoric flood that mowed over and buried a stand of willows growing along a small stream.
The first attention-grabber in the tunnel is the smell -- a sour, musty stench that stomps into the nostrils and stands its ground. It is the smell of old plant and animal life in slow, steady decay caused by exposure to air.
''It's like opening an outhouse door,'' said Jim Beget, a UAF geologist in the midst of his second climate-related study using the tunnel. ''It's unmistakable when you go in there.''
At a glance, the tunnel looks like just any old cave. But closer inspection reveals the ice in the stratified permafrost -- glints of tiny crystals in some spots, large disks and wedges with the look of worn glass in others.
Not far inside is the most obvious fossil find -- a tooth-filled jaw and other brownish bones from a extinct long-haired steppe bison that have been locked into the permafrost wall for some 14,000 years, according to carbon-dating techniques.
Elsewhere in the tunnel, remains of snail shells, beetles, moths and other tiny creatures can also be seen.
The tunnel's ceiling is a dozen feet tall at the entrance, tapering down to less than 5 feet high at the back. Willow logs stick out and slender plant roots clotted with dirt dangle from the ceiling like moss from a bayou cypress.
Deep into the tunnel is a cross-section of an ancient pond frozen down to the reddish, iron-rich deposits resting on its floor.
A passageway leads into a dark chamber carved out of the middle of that pond. Boone describes the sensation of being in the ice room as both exhilarating and a little eerie.
''It's this fantastic feeling of being in a place where humans are not supposed to be,'' he said.
A thick layer of powder-fine silt on the floor is easily kicked up into a dusky haze. The silt, which readily clings to the walls and ceiling, was blown north from Alaska Range glaciers during the last ice age, Beget said.
For several years Beget and a Japanese scientist have been sampling air bubbles trapped in permafrost ice to learn about the air's composition as far back as 35,000 years.
''The bubbles reflect atmospheric conditions and tell us something about the organic material on earth's surface,'' Beget said. ''We can look at the methane content (in the air) to determine whether there were plant communities then that were different than now.''
Such measurements could provide some insight into the extent of global warming in the polar regions, he said.
The uniqueness of the permafrost tunnel has attracted the interest of Fairbanks-area tourism operators, but the Army Corps has no plans to hand over the keys.
''The actual state of the permafrost is very delicate,'' said Maj. Eric Wahlgren, who heads the cold region lab's Alaska office.
''The number of people going in and out would cause a temperature fluctuation that could damage some of the artifacts and things being studied.''
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