Another characteristic of Rimonio, linked to the presence of limestone, concerns the phenomenon of karstism.
Karstism (a term derived from the Karst region of Trieste, where the phenomenon was first studied) indicates the chemical activity exerted by water, especially on limestone, either through dissolution or precipitation, which leads to the formation of dolines, sinkholes and underground cavities. This phenomenon is also evident in our Rimonio where, at a certain point, the water filters into the subsoil and resurfaces at a much lower level.
The Apuan Alps are one of the largest karst areas in Italy, with very long and deep caves. The Apennine part is affected by minor karst phenomena in the Val di Lima and in the areas of Pania di Corfino and Alpe di Soraggio.
On the right orographic side of the Rimonio there is the Buca delle Fate (name given to many caves in the past, when the underground world was thought to be inhabited by supernatural beings). This cave, which has been known to the local population for a long time, was explored in the 1960s by the Gruppo Speleologico Bagni di Lucca and later by the Speleoclub Garfagnana, which surveyed it and cared for its census in the Caves Register of Tuscany under No. 1256 T/LU. It is 64 metres long and 17 metres deep.
It is strictly forbidden to enter the cave unless accompanied by speleologists.
On the opposite side of the cave's opening, the ruins of the old Buca delle Fate Mill are visible.
