Geological studies of the Sibillini Mountains carried out mainly during the last century, provided evidence of a hypogeal karst characterized by a small number of caves of limited extent. The only one mentioned by numerous ancient authors is the ‘Grotta della Sibilla’, on account of its legendary references. This cave is the keeper of one of the most fascinating secrets of the Apennines, having been both a place of mountain cult as far back as pre-historical times and the home of the fortune-telling prophetess ‘Sibilla’. Historical sources tell of the presence of someone mysterious at the site from the time of the Romans but amongst the historical descriptions, the testimony of Antoine de la Sale (photo of his map) is most notable: he visited the cave in 1420 and described it as a good-sized cavity within the bowels of the mountain. Nothing about this setting is mentioned in the geological literature or in topographic descriptions, made for the first time at the beginning of the 1940s, when a regular but small cave was revealed. Today rockfall deposits completely obstruct the entrance.
On the basis of the above-mentioned legendary references, geomorphological and geophysical studies started helping to define the real extent of the cave. The planimetric trend of the electromagnetic anomalies surveyed allow us to make hypotheses about the presence of a vast hypogeal system.
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