The Buca delle Fate, a natural ravine between the rocks on the slopes of Poggio di Montieri, is a medieval tunnel opened in the Jaspers (red siliceous rock) in correspondence with a fracture with a N20° direction which would have facilitated its excavation. The tunnel has a significant slope (25°) and is very short, about 14 metres; in its terminal part there is a 2x1.5m well currently closed by debris. In the deepest part of the tunnel, in the vault of the well, modest traces of newly formed minerals were found consisting of crystalline concretions of calcite (calcium carbonate), spangolite (hydrated chlorine sulphate of copper and aluminum), nantokite (copper chloride) and brushite (hydrated calcium phosphate).
At the top of the well you can observe quadrangular recesses and indentations dug into the rock which were used to anchor the support beams of the winch. Since the tunnel is very narrow (50cm on average) it has two exchange niches along its path, useful for allowing the transit of two miners.
Other small rooms used to place lamps are also visible. The absence of evidence of mineralization and an external mining dump makes it implausible that this is a mining tunnel.
It is reasonable to hypothesize that this work was functional to the vast underlying excavations, identified in the Santa Maria dei Castagni tunnel between the 19th and 20th centuries. and therefore used as a secondary access route to mining works or as a ventilation duct.