The Lago di Venere ("Lake of Venus") fills a flat-bottomed volcanic crater on the north side of the island, surrounded by green hills that come as a slight shock after so much black lava. The water is alkaline and warm, fed by hot springs from below, and the muddy banks are slathered on by visitors as a free spa treatment.
You can walk around the lake in about an hour, or wade in and float for as long as the wind allows. The mud is best applied in the morning, left to dry in the sun, and then rinsed off in the lake itself. There is no entry fee and no real facilities — a tiny shaded car park, a couple of food trucks in summer, and the bird calls of moorhens in the reeds.