The Smithsonian Channel will air the documentary “Americans Underground: Secret City of WWI” made by the South Portland production company Lone Wolf Media.

The film, which airs at 8 p.m. Monday, tells the story of the discovery of a vast underground city beneath a farm field in northern France where soldiers on both sides of the conflict in World War I took refuge a century ago. It is one of hundreds of buried havens that were created close to a 45-mile stretch of the Western Front.

In “Americans Underground,” explorer and photographer Jeff Gusky documents one of these shelters, and attempts to connect the names of the American soldiers etched into the limestone walls to their living descendants.

With help from military experts and historians, including Jonathan D. Bratten, the command historian for the Maine Army National Guard, and Passamaquoddy author, filmmaker and historian Donald Soctomah, Gusky traces the etchings back to soldiers from the Yankee Division, an American unit from New England that was among the first to arrive in France.

It included soldiers from Maine, Massachusetts, Vermont and New Hampshire. As he unlocked mysteries hidden in the underground city for a century, Gusky discovered intriguing American Indian images and symbols that led him to stories about the Passamaquoddy Tribe from Maine.