The last veterans of World War I died a few years ago. When we think of that war today, we imagine an age of Edwardian sensibilities, ragtime music and people with little connection to our modern world.

But as Jeffrey Gusky, an American emergency room physician, explorer and photographer discovered, the soldiers of that era were more like us than we may think. And they left behind many reminders of their existence, hidden away in places we never knew existed — the underground cities of World War I.

Essay A continuing exploration of World War I as seen through photography. Photographers on the Front Lines of the Great War

Ancient rock quarries that provided the stone that built the castles, fortresses and cathedrals of France lie beneath many of the trenches dug by the invading armies of Germany and its allies, who faced the French, the British and, later, the Americans. Engineers connected the trenches to these quarries — used by all sides — which became staging areas, hospitals, canteens and shelters from artillery bombardments. Some even have street signs and maps.

“When you’re underground in these places, and it’s completely dark, your headlamp shines on a particular part of the wall and there, looking back at you across a hundred years, is a signature of someone that looks like yesterday,” Mr. Gusky said. “It’s a very deeply moving thing because you feel like someone wanted to be known, they wanted to say, ‘I existed, I mattered, I was alive.’ ”

Yet a century since the war’s outbreak, the French — like many others — had forgotten their very existence. Mr. Gusky was working on a photography project in France when he happened upon one of these underground cities, carved from stone and occupied by soldiers seeking safety as hellish battles raged aboveground.

As he began to explore and photograph these places, Mr. Gusky felt a connection to the men who lived and died there. As he studied at the carvings and graffiti the soldiers etched into walls of these caverns, he realized that the lives the soldiers left behind in their hometowns were fundamentally the same as our own.

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“They lived in high rises, they drove cars, they went to work on subways, they went to the movies, and they were bombarded by mass media and messaging just like we are, and it was very sophisticated,” he said. “By finding a hidden world that haunts the present and photographing it — as far as I know, this is the only direct human connection between then and now. And for the vast majority of people who are not interested in World War I, who feel no connection to it, this is a way to make it real and make it relevant.”

In one of these places, occupied by the French, three chapels — two Catholic and one Jewish — exist. A carving depicts a French soldier kneeling in prayer. In another, bottles and canteens of water sit at a table, as if a meal had just been finished.

In some places, looters have stolen objects and, in some instances, removed entire sections of walls with war carvings. In one case, Mr. Gusky was looking for an American ambulance carved into a wall. After three attempts to reach it, crawling over jagged rocks and through collapsed spaces, he came to a big, empty space on a wall where people went in with special rock-carving equipment and cut it out from the wall.

With these underground cities protected by landowners who often conceal or put bars over the entrances to deter looters and vandals, Mr. Gusky had to work to gain their trust. He gained access to dozens of sites, despite not speaking French and most of the landowners not speaking English.

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“The photographs were a language that we shared, and a passion to preserve a legacy,” he said. “And they began to share their secrets.”

The challenges of photographing in complete darkness underground are many. “We learned how to create context because these places are vast spaces,” he said. “If you just photograph, for example, a sculpture and you don’t light what’s around it and a hundred feet away, then you don’t get a sense of a space.”

Mr. Gusky sees parallels between his work as a rural emergency physician and as a photographer. “There’s a realness to the experience of rural emergency medicine that I bring to the photography,” he said. “It’s a journey through darkness to light, and always toward hope. I don’t do photography for fun; it’s not a hobby. It’s a very serious thing for me, and it’s a way to talk about today and about us without being right or left. And World War I offers a way to talk about the way that modern life affects our humanness without being political.”

Mr. Gusky said the soldiers who lived and died in these places have a message for us. “They’re almost begging us to ask the questions about modern life that we’ve forgotten how to ask,” he said. “How it affects our humanness. And we see on these walls a struggle to remain human in a world that had become inhuman.”

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