Lerma is situated on the boundary of Capanne di Marcarolo Natural Park, 8100 hectares stretching from Ovada to Ligurian Apennine mountains.

Il Mulino di Lerma lies in the wood of Spinola Medieval Castle, standing on a rock under which the Piota stream forms a bathing lake. The water is pure and crystalline and comes down from Ligurian inland uninhabited and wild mountains.

The lake is from less than 50 centimeters to a few meters deep; it is possible to plunge from the rocks.