These horseshoe-shaped mounds, typically found near water, are the burnt and discarded residue of a Bronze Age cooking method: stones were heated in a fire, dropped into a water-filled trough to bring it to a boil, and then, cracked and spent, piled to the side. Over centuries of use, those rejected stones accumulated into the mounds that survive today, often the only visible trace of activity stretching back three or four thousand years. Ireland has thousands of them, making the fulacht fia one of the most common ancient monuments in the country, yet the people who used them, and precisely what they cooked or processed, remain subjects of ongoing debate among archaeologists.