As soon as I parked the car along the steep roadside, Lucas said,
"What's that stink?!"
The rotten egg smell helped Lisa guess... natural hot springs. The Bagno San Filippo have been there for eons... Roman Caesars and popes have bathed in their sulfur hot waters. Since we parked our car right above the first 100 foot tall stalagmite-like snow white formation (streaked with colors of other minerals), the steamy stench made Lucas pull up the neck of his shirt over his suffering nose. It really smelled like rotten eggs. He decided maybe this was a good cover for his farts. I couldn't tell.
The best thing about this site is that it's free!
We hiked down into the Fosso Bianco gorge and found this formation... and a much larger one beyond--two tiered and stepped. Picture white cream puff mountain steaming. Streaks of colors here and there. We didn't go in ourselves but the water was like a warm bath--some areas are much hotter than others. Many local Italians bathe here for curative effects.
The area is pretty big, and the first pools you find along the path near the road are shallow and tend to be a bit cooler than ones further downstream. The first formation is massive--Il Ghiacciaio (the Glacier). If you follow the path further on, you reach the immense formation called Balena Bianca (White Whale). The water tends to have a milky, bluish color in places. The area has many pools to sit in, but what I heard is you'd better wear old bathing suits because you will never lose that stink. It won't wash out.
We talked to an Italian couple who were bathing there and they said it the waters weren't as hot as a few months ago in summer. There is also a proper spa in town that gets the waters piped in directly from these springs.
Remember, this is little town also has Spa Terme San Filippo, the main attraction with a hotel, restaurant, pool, spa and wellness center, but if you're inclined to "take the waters", do as the locals do, and just take a walk down into the free park area.
Italy is a volcanic land. This was a fantastic off the beaten path sight to see. This country is full of natural wonders. The geology of the place staggers me.
Next... later in the day, a castle tower on a mountaintop...