It began quietly, on an April morning in 2020, when the couple—identified only as Mr. and Mrs. W.—were adjusting a fence beside one of their backyard flower beds. While digging around a post, Mr. W. noticed something odd: a lump of “clayey soil” containing thin metal discs. He placed it aside, thinking little of it.

But curiosity has a way of traveling from person to person. Later, their teenage son — intrigued by the mysterious clump — rinsed it under the garden tap. As the water cleared the dirt, sunlight caught a sudden glint.