The tarrasque is one of the most iconic monsters in D&D, and its origins are mysterious and varied across settings. In Forgotten Realms, its origin is unknown but the center of speculation, and in Pathfinder, it's the herald of the apocalypse god. Its current location is also generally up to GMs--in Pathfinder, for example, it's "in a cave somewhere in Avistan", but I've put it under a mountain and in a homebrew Hell. We've all seen the story of the city that keeps it trapped and harvests resources from it, and I and a friend both make a point of hiding it somewhere in every homebrew world we create.

So, fellow worldbuilder DMs, what's your lore? Where did it come from? Where has it gone? And why is it so gosh darn unkillable?