These sites, known in Irish as fulachtaí fia, appear in their thousands across the country, typically as low, horseshoe-shaped mounds of fire-cracked stone and darkened soil. The leading theory is that they were used for cooking or heating water, perhaps by dropping stones heated in a fire into a water-filled trough until the liquid boiled. They cluster around bogs, streams, and low-lying ground, and most date to the Bronze Age, roughly 1500 to 500 BC, though some are earlier or later. The sheer number of them, and the stubborn uncertainty about exactly what they were all used for, gives each one a particular kind of interest.