The native people of the Lake Superior region, long gone by the time the Ojibwe arrived from the East hundreds of years ago. Their civilization retreated to a number of underground towns for an unknown reason, and then vanished. The towns are marked by concentric, geometric chambers. Much of the copper they mined is still in circulation as the copper pieces of the Nords or the Penni of Isle Royale.
The Plano were skilled miners, and were rumored to have retreated below ground to hide from a long, cursed winter - some sort of punishment from a powerful and very angry manitou.