A large part of the historical buildings in central Italy have used volcanic rock in their construction. These materials, however, have seldom been used for artistic purposes. An interesting exception can be found in the Bomarzo Park where a large number of sculptures have been carved in the local trachytic rocks. The environment created by the natural outcrops and the easily carved rock, suggested the realization of the sculptures directly from the dispersed boulders. These sculptures, which date back to the 16th century, are suffering from alteration processes mainly related to the high humidity of the site. The causes of these alterations and the present condition of the stone have been studied with the aim of recommending suitable conservation procedures.