Hi Mugball,
Our first weaners went on land that unknown to us, the previous owner used to burn absolutely any rubbish going. The pigs dug up everything from broken glass, melted glass, old rusty bedsprings, hinges, nails, screws, farm tools and all sorts of other items. As other have said, they didn't come to any harm but we cleared up daily after them. To this day they still occasionally discover something new!
Pete