It is a fulacht fia, one of thousands scattered across the Irish landscape, and it represents one of prehistory's more quietly fascinating puzzles. These burnt mounds, as they are also known, typically consist of heat-shattered stone accumulated around a trough that was once dug into the ground near a water source. The standard interpretation is that stones were heated in a fire and dropped into a water-filled trough, bringing it rapidly to the boil. What the boiling was actually for, however, has kept archaeologists arguing for decades. Cooking meat is the most widely accepted explanation, though brewing, textile processing, and bathing have all been seriously proposed.

Fulachtaí fia are overwhelmingly Bronze Age in date, with the majority placed somewhere between 1800 and 800 BC, though some sites have yielded earlier or later dates. They tend to cluster in low-lying, marshy ground, which fits the practical requirement for a reliable water supply close at hand. Mayo, with its boggy terrain and high rainfall, has a considerable number of them, and many survive well precisely because blanket bog has grown over and preserved the charred, fragmented stone that gives the monuments their characteristic dark, humped profile. The site at Lack is one such survival, recorded but so far sparsely documented in the public record.