The U.S. Army Permafrost Tunnel

Between 1963 and 1965, the U.S Army Permafrost Tunnel was dug "entirely within frozen ground on the north slope of Hill 456 near Fox, Alaska."

"Initial research" at the site "focused on developing new mining and tunneling methods for building underground facilities and foundations in permafrost. Special emphasis was given to tunnel behavior in permafrost, including deformation, natural air flow, feasible types of ventilation and thermal regime."

The tunnel is now maintained by the U.S. Army Corps of Engineers' Cold Regions Research and Engineering Laboratory – whose work "includes an amazing array of topic areas, including engineering and technology in cold regions, seismic-acoustic physics, tools for military combat and survival in cold weather, and many others."

If you're hoping to explore the place in person, however, beware that the tunnel's structure "requires a freezer unit" in order to survive the Alaskan summer – lest the whole place spontaneously liquify...
Meanwhile, soon to be discovered: a whole city of post-military ice caves, carved throughout the Alaskan peninsula and populated entirely by sightless humanoids – who are victims of an Army gene experiment gone awry.
Starring Samuel L. Jackson as a hot-headed spelunker who discovers his own father living nude in the deepest tunnel. "Enough is enough!" Jackson yells. "I have had it with my motherfuckin' father living in this motherfuckin' cave!"