Piper

The Piper is the largest of the Dancers, the standing stones just outside Lancre. They are believed to have fallen out of the sky for no readily apparent reason several thousand years ago - although Nanny Ogg is on record as saying that the sky is not a sustainable environment for large rocks, as a rule, and of course they'd bloody well fall, Well Known Fact, if anyone put them up there.

They delineate and quarantine a natural portal between the Discworld and the world of the Elves. They contain a lot of the metal which loves iron, to such an extent that nothing made of metal may go inside the ring; indeed, the very hobnails are torn out of Granny Weatherwax's boots when she tries. A flatiron on a rope, however, has sufficient velocity to make its own rules concerning mere magnetism, and in the case of one luckless elf, the velocity is near terminal.