World War I began a century ago this year in Northern Europe, eventually claiming the lives of 10 million soldiers and more than 100,000 Americans.

On Veterans Day, America focuses on the heroes of the five wars the nation has fought since then. But Dr. Jeff Gusky, a Dallas physician and photographer, has spent the last four years re-discovering caves in Northern France that have much to tell about the men who fought World War I.

World War I is remembered by many as the conflict that brought industrialization to killing. Gas warfare. Mass casualties. And immobilizing trench warfare, where men were sapped of their will to live as well as their will to fight.

What's less realized is what happened underneath the trenches.

"While World War I was a trench war, it was also a war of underground cities," Gusky explained.

These underground cities — some housing as many as 24,000 soldiers — were located in hundreds of caves and stone quarries that predated World War I by hundreds of years. Most are on private property, and were occupied by armies of both sides, with each side living in caves closest to their own trenches.

"They became underground cities with the technology of the day," Gusky said. "Rail, telephones, electric lights and power stations. Hospitals, food systems, plumbing. There are even street signs — they are that big and that complex."

More amazing than the technology is the art, as Gusky reveals in hundreds of stunning photographs. If art is a voice, the men of World War I are still speaking to us.

They carved memorials to their lost friends. Their hometowns. Their girlfriends. The nation's Icons.

"What you see is the way modern people hold on to their humanness at a time when the world is becoming terribly inhuman and incomprehensible," Gusky said. "You find chapels and synagogues, theaters... it's a remarkable capsule frozen in time."

Merciful heart of Jesus have Mercy is scratched into one wall. The 101st Infantry roll of honor is carved into another.

"I can feel the presence of these guys when you see their streets listed, and when you see the notes to loved ones," Gusky said. "There's one that comes to mind, and it's the profile of an American face. And there's a flag on the face, and there's a tear in the eye. He's missing home."

More than 116,000 American soldiers never made it home. Gusky found a private gravesite containing more than 7,000 soldiers of all armies on private property in northern France.

"You see a scale of killing that manifested itself in different ways, throughout the 20th century and today," Gusky said. "It's the inhuman scale, where the meaning of life doesn't matter."

As the world has moved on to other conflicts, the land of Northern France still remembers World War I.