YPRES, Belgium — The padlocked cage beside the driveway on the Butaye family farm near this town in western Belgium is almost full of rusting bombs again. Since January, Stijn Butaye has collected 46 mortar shells on his family’s 100 acres, World War I munitions he found among the sugar beet and potato fields, sometimes with the help of his metal detector.

Mr. Butaye’s father, Luc, won’t even plow two of his fields for fear of what the blades might hit. Not long ago, a neighbor riding his tractor ruptured an aging shell, and the explosion sent shrapnel through his windshield, tearing off a chunk of his ear.

“You don’t know what could happen,” said Stijn Butaye, 26, who has built a small museum beside the barn with hundreds of items — including shoes and eyeglasses and razors and a perfectly preserved gas mask — that he has found on his family’s property. “We just use that land for grazing the cows.”

It has been 100 years since World War I erupted in these parts. The men who survived the thousands of miles of muddy trenches that surrounded this strategically important region are long gone and buried. But the earth, in its own way, has become the last witness, coughing up constant reminders of a bloody and relentless war that would demolish empires, leave at least 8.5 million soldiers and seven million civilians dead, and produce legacies that continue to play out today.