The well at Knocknaganny, on the edge of Sligo town, carries a rather different kind of foretelling: that it will one day rise up and drown the entire town beneath it. This is not a holy well in any recognised sense, and there is no tradition of pilgrimage or pattern days associated with it. It is something older and stranger, a water source tied not to sanctity but to enchantment and slow, inevitable doom.

The well's Irish name, Tobar na Seilmide, translates simply as Snail's Well, and the reason for that name is where things become genuinely peculiar. Writing in 1836, Thomas O'Conor set down the local belief as it was then understood: once every seven years, a snail emerges from the well, and this creature is held to be an enchanted being, a metamorphosed entity of some kind, whose presence is connected to the well's eventual catastrophic overflow. The prophecy, as O'Conor records it, is quite specific in shape if vague in timing; the snail appears, the waters will rise, and Sligo will be overwhelmed. The well appears by this name on the first edition Ordnance Survey six-inch map, which places its formal recognition as an antiquity in the nineteenth century, though the belief surrounding it is presumably older. What is notable is what the well is not: unlike the vast majority of named wells in Ireland, it has never been classified as a holy well, nor does it appear in historical or antiquarian literature in that context. It belongs to a different category of folklore entirely, one concerned less with healing than with warning.