These features, known in Irish archaeology as fulachtaí fia, are low, horseshoe-shaped mounds of fire-cracked stone and charcoal-blackened earth, the accumulated debris of repeated heating. The working theory, supported by experiment, is that stones were heated in a fire and then dropped into a water-filled trough to bring it rapidly to the boil. What the boiling was actually for, whether cooking, textile processing, bathing, or something else entirely, remains genuinely contested among archaeologists.