On the shores of what we call the Great Slave Lake lived the Slevs who created the now familiar Vezhda Caves.  These were manmade structures with only one door and no windows.  Even this door would often be locked or guarded.  Inside would be a beautiful abstract painting or carving visible—even during the day—only by torchlight.

Quite a few these survive, though the Slev empire was relatively short-lived.

You can see them all over the northern Great Plains, like the heart of Northern Kymaerica, non-descript boxes with magic inside.  Many believe that they began as a result of stories of legendary dwellers among the Slev before they came to the shores of the lake.