The entire promontory is made up of piles of limestone debris that over time have undergone karstic and marine erosion, forming numerous caves with stalactites, stalagmites, underground ponds, tunnels and tunnels that were inhabited in the Neolithic era. The following can be considered of considerable historical interest:
Grotta grande dell’Argentarola, underwater cave located in the islet of the Argentarola, relevant site in the research conducted for the study of the climate and in particular the changes that have occurred in the past to which lowering and sea level rises are related .
Grotta dei Santi, in the homonymous bay, opens on the south-eastern coast of the Argentario and is characterized by the presence of prehistoric finds studied since the fifties of the nineteenth century. It is the permanent site of an excavation campaign involving scholars from the University of Pisa, Siena and the University of São Paulo in Brazil.
Grotta degli Stretti (of the Grand Duke) or Grotta di Punta degli Stretti, located along the Provincial Road 161 that from Orbetello leads to Porto Santo Stefano, discovered in 1842 is characterized by the presence of inland lakes with numerous rooms and galleries, frequent excursion destinations. In it were found remains of bones and tools of the Neolithic age.
Also worth mentioning is the Grotta del Turco, along the coast of Cacciarella, the Blue Grotto, near Punta Avoltore, the Grotta di Nonna Vittoria, located in Punta della Riccia and the Grotta della Polveriera in Poggio Mortaio, where a Tirreniana beach has been found.