Think about the huge stone buildings of France — Notre Dame or the huge medieval castles. That stone had to come from somewhere, right? In fact, it came from huge underground quarries, some under what would become the battlefields of World War I.

During that war, soldiers on both sides took advantage of those quarries, now abandoned, and in the largest of them they built what can only be called underground cities, with room for thousands of people inside and facilities that you probably wouldn’t associate with the front.

The quarries were an escape from the absolute horror of trench warfare, where a soldier could relax, go to church, maybe even visit a bakery. Many of them carved names, symbols or even sports scores on the walls of the caverns.

In some places, the quarries are more or less intact — and more or less forgotten, except by the locals whose land is above them.

Photographer Jeff Gusky learned about the quarries while in France on another project and discovered among other things that some of their most prolific decorators were the men of the 26th Yankee Division, many from Maine.

Gusky’s search for those men’s stories brought him to Maine. That trip is documented in the film “Americans Underground.” Nora Flaherty of Maine Public spoke with Gusky via Skype about the project and about the scale of the underground cities.

Gusky: I’m tall, almost 6-foot-4, and I can walk in these spaces and not feel claustrophobic. They’re huge. They’re like subway stations, like modern cities. Lights, telecommunications, rail, hospitals, food systems, ventilation systems, sanitation, offices, living quarters, theaters, chapels, synagogues, the works. Some of these places are so big there are street signs.

What’s hard for us to get our minds around as modern people is that there would be something like this that could exist and not be on the internet, not be known. And yet it exists.

Flaherty: This is just absolutely fascinating — and the visuals are just amazing. But you say it also brings a sense of immediacy to something that seems like very long ago.

Gusky: World War I is remembered through old photographs and silent films that make it seem ancient. And yet these were modern people fighting a modern war.

And there is just stuff everywhere. When the war ended, they wanted to go home, and so they left uniforms and helmets, there’s live munitions everywhere, hand grenades and live shells and bullets, cooking implements and clocks. You just find lots of stuff.

Flaherty: And people didn’t just leave their stuff. They also left their marks on the walls of the quarries.

Gusky: On the walls of these cities, you see the inner life of these soldiers, you see expressions of their humanity. Notes to loved ones, prayers to a higher power. You see the emotion of missing home, the love of America.

And you see jokes, you see sports scores — for example, the Red Sox were on a winning streak in 1918, and as we know now, the next year was the beginning of the Curse of the Bambino, and they wouldn’t win again until modern times. But these guys didn’t know that, so in big bold letters, under a French farm field, you see “Red Sox 7, Yanks 4.” That was a real game and they were so proud of it.

Flaherty: Another thing you found on the walls were symbols that you couldn’t identify, and you eventually determined they were made by members of the Passamaquoddy Tribe from Maine. You knew there were six Passamaquoddy troops from Maine who’d served and been in the quarries, but the connection wasn’t completely clear. How did you find out?

Gusky: One of my team who has proved to be an invaluable part of the project, Lt. Jonathan Bratten, the chief historian for the Maine National Guard, a really smart guy, he knew there were nine Passamaquoddy young men, who volunteered the week after we declared war, even though they were not citizens. (They became citizens with the Indian Citizenship Act 1924, or the Snyder Act.) They couldn’t vote, they didn’t have the same property rights, there were a whole list of things they couldn’t do because they weren’t American citizens. But they loved America. They were devoted to their country.

So when he found out they were down there, he went up to the reservation at Pleasant Point, and he wanted to find out if there were any families he could talk to that had sons among this group that volunteered.

He was talking to the tribal historian, and he happened to have some of my photos on his iPad, and he showed them to the historian, and he saw something none of us were seeing: ancient tribal symbols dating back thousands of years that stood for peace and friendship. Maybe four or five Native American carvings, that in fact are Passamaquoddy and we know that because of the symbols they carved that were distinctly Passamaquoddy.

This interview has been edited for clarity.

This article appears through a media partnership with Maine Public.