Ruins of Bains de la Caille
The Bains de la Caille (Baths of La Caille) are former thermal baths, now lying in ruins at the bottom of Gorge des Usses below the Ponts de la Caille, between Cruseilles and Allonzier-la-Caille in Haute-Savoie. There have been baths on the site since the middle ages, because of the presence of a source of sulfur water. However, the modern Bains were built in the mid-19th century. It drew customers from Annecy and Geneva and there was extensive construction done over the years, including a hotel, a church and even a casino. The Bains closed for good in 1960. Most of the buildings have been dismantled it seems but, for some reason, the building containing swimming pools is still standing today. There is also a small metal bridge (which is kinda falling apart) to cross the Usses river near the swimming pool building, as well as ruins here and there on both sides of the river.
Visiting the Bains is officially forbidden and dangerous (apparently there is a risk of rockfall from the cliff above) but the ban is not really enforced and the path is not blocked very convincingly. The most direct way to come is by hiking down from the Ponts de la Caille (Bridges of La Caille), on the Allonzier side: There is a steep road with many switchbacks that goes to a large bridge over the Usses and then to the Bains nearby. However, erosion over the years and lack of maintenance have transformed it into more of a trail (barely passable at one point). There is also an easier path from the mouth of Gorge des Usses: This is the one I took on the way in, hiking from Copponex (I used the direct road to the Ponts on the way out). I will post more details about that hike in a few days.