The Sibillini Mountains, or Sibylline Mountains (Italian: Monti Sibillini) are a mountain group in Italy, part of the central Apennines. Situated between eastern Umbria and the Marche, they are mostly composed of limestone rocks, formed in the Mesozoic and Cenozoic (100 to 50 million years ago) on the bottom of an extinct sea. The land emerged 20 million years ago.