It’s not exactly a science fiction time tunnel, but it’s the next best thing.
About 16 kilometers (10 miles) north of Fairbanks, Alaska, in the old gold mining town of Fox, sits a refrigerated passageway to the past. This "time machine" is an underground tunnel into 44,000-year-old permafrost (permanently frozen soil). It is 110 meters (about 360 feet) long and four meters (about 13 feet) in diameter.
Mammoth bones, bison horns, grasses, and sedges are visible in the frozen walls of the excavation, offering a unique view into a time period that was vastly different from today. This underground passageway is the Permafrost Tunnel belonging to the Cold Regions Research and Engineering Laboratory’s (CRREL).
Originally excavated as an Army engineering research project in the mid-1960s, the Permafrost Tunnel now serves as a basic science and permafrost engineering research platform for studying the formation of permafrost, its structure, recent climate history, how frozen soil deforms with changing temperature, and plant and animal remains.
It can even be used to study Mars…more on that later.
Permafrost is soil that has been permanently frozen for two or more years, commonly found in high latitude regions such as Alaska, Canada, and Russia. The Fairbanks area is a region of discontinuous permafrost. Hilltops and southern slopes are usually free of permafrost, while northern slopes and low areas are permanently frozen.
The permafrost layer here extends about 30 to 50 meters (about 98 to 164 feet) below the surface, but it can exceed a thickness of 100 meters (about 328 feet). The soil froze during the Pleistocene Epoch, about 1.6 million to 10,000 years ago, spanning the most recent ice ages when a host of plants and animals thrived in very different climatic conditions than today.
Interestingly, during that cold time the interior of Alaska was not glaciated and the visible animal remains suggest a rich animal habitat.
The tunnel contains features typically found in fine-grained permafrost, such as ice wedges, segregated ice, and erosion boundaries, all readily seen in the walls and roof. Ice wedges are one of the more fascinating features, and the Permafrost Tunnel has many good examples in various stages of development.
Ice wedges are created when the surface of the soil contracts during cold winter temperatures, forming cracks in the soil. A crack can extend down into the soil many meters, and can run along the surface many times that length. In the spring and summer, surface melt water fills this narrow crack, and then the water freezes during winter. Repeated thawing and freezing cycles for thousands of years creates a wedge of ice meters in width at the surface extending far down into the soil.
Another interesting ice feature is segregated ice, which occurs while the soil is in the process of freezing. As the freezing front moves downward in non-frozen soil, and if a source of water is available to continually feed the freezing front, a horizontal layer of ice will be created ranging from less than an inch thick to several inches. Segregated ice is essentially the same process as frost heaving, which is experienced with roadways and soil in mid-latitudes in the winter.
The Permafrost Tunnel began with a vertical silt escarpment created by gold mining in the early 20th century. It was completed by the U.S. Army Corps of Engineers using a continuous mining machine called the Alkirk Miner during the winters of 1963-64, 1964-65, and 1965-66.
The horizontal section (adit) is 110 meters (about 360 feet) long and predominantly located in the silt. The declined section (winze) begins just inside the portal and drops at a gentle slope for 45 meters (about 885 feet), passing into the gravel and then into the weathered bedrock.
The ambient temperature of the permafrost in the tunnel is about one degree below freezing, and this is too warm for the frozen silt to support the weight of 14 meters (about 46 feet) of soil above. So forced cooling is used to keep the interior temperature at four degrees below freezing or colder. Mechanical cooling is used for the summer, and ambient cooling (outside cold air) is used during the winter.
These cold temperatures give the frozen silt the strength needed for self-support, and allows an unobstructed view of the interesting features in the walls and roof.
The Permafrost Tunnel has many benefits as a research platform. In the open air during summer, permafrost remains frozen only for a short time. Winter study is limited by extreme cold and darkness. So a lighted, steady-temperature facility allows precise measurements and sample retrieval that is not possible otherwise.
Some research has been conducted on the different soil types and the massive ice they host, and some studies have examined life that might have existed thousands of years ago. Recently, dirty ice was taken from a few of the ice wedges, and 25,000-year-old bacteria were cultured, giving researchers a look at unique organisms from the past.
The Permafrost Tunnel may even give insight into soil processes on Mars. It is now almost certain that Mars once had an atmosphere and flowing water. But the water froze as the planet gradually lost its atmosphere and it is possible that Mars now has vast layers of deep permafrost. A project is underway where locations in the tunnel are similar to features photographed on Mars, which may give researchers an up-close look at a very faraway place.
Also, permafrost is a key environmental factor when considering resource extraction in the north. The Permafrost Tunnel has provided an excellent test bed for geophysical research, and this continues today.
The tunnel has not always been used for scientific research. Its original purpose was military engineering. In the mid-1960s the U.S. and Soviet Union were in a Cold War, and the tunnel was excavated to determine if expedient underground excavations in the north could be used for military purposes. So an excavation was done with speed and efficiency in mind. They used the Alkirk Miner with two large cutting disks for its nearly continuous excavation ability, similar to modern tunneling machines.
After the Permafrost Tunnel was complete, researchers recognized the benefit that could be served to advance permafrost understanding. The tunnel remains one of the only facilities specifically used to research permafrost soil and ice. Scientists and engineers today would like to expand the tunnel to uncover new features and views, so Alaska District is working with CRREL to develop preliminary plans on how the tunnel could be expanded to better facilitate research and outreach.