The "Sass da Preja Buia" is a big erratic boulder (Masso Erratico) on which are inscribed symbolic petroglyphs that suggest it was a sacrificial altar from prehistoric times.

All around there are other erratic boulders, on which are present other petroglyphs. The Boulder dates back to the last Neozoic glaciation. The ice, in its progression, dragged both moraine material and large boulders, which literally wandered (errare) into the countryside.

In the surroundings of Lake Maggiore we can find other famous erratic boulders, such as the Sass Cavalasc in Ranco and the Sasso Galletto in Castelveccana, which rises directly from the waters of the lake.