The territory of Velletri stretches between two distinct areas. The northern part is situated on the southern foothills of the Colli Albani range and was geologically formed about 150,000 years ago, after the collapse of the Volcano Laziale (caldera). The southern boundary forms around Pontine Marshes, whose reclamation started at the time of Pope Pius VI and was accomplished during the regime of Benito Mussolini.