In Lough Brohly, County Mayo, that is precisely what happened to a small island believed to be a crannog, the kind of artificial or artificially enlarged island that early medieval communities built in Irish lakes as defensible homesteads. The island, roughly circular and covered in vegetation, measured somewhere between 35 and 40 metres in diameter, and sat close to the centre of the lake. It is visible on the Ordnance Survey six-inch map of 1838, placed there with the quiet confidence of a surveyor who could see it. On later editions of the same map series, it is simply gone.

The most likely explanation is not that the island was destroyed or removed, but that the water level in Lough Brohly rose over time, gradually submerging what had once been visible above the surface. Crannogs were often built just barely above the waterline, meaning even a modest rise in lake levels could render one invisible without eliminating it altogether. The underlying structure may still be present beneath the surface, waterlogged and largely intact, as is the case with many submerged crannogs elsewhere in Ireland. The 1838 survey gives us the clearest fixed point: at that moment, the island existed as a tangible, mappable feature, roughly in the middle of the lake, with enough vegetation on it to be identifiable.