The Pontine Marshes is an approximately quadrangular area of former marshland in the Lazio Region of central Italy, extending along the coast southeast of Rome about 45 km from just east of Anzio to Terracina, varying in distance inland between the Tyrrhenian Sea and the Volscian Mountains from 15 to 25 km. The northwestern border runs approxim…The Pontine Marshes is an approximately quadrangular area of former marshland in the Lazio Region of central Italy, extending along the coast southeast of Rome about 45 km from just east of Anzio to Terracina, varying in distance inland between the Tyrrhenian Sea and the Volscian Mountains from 15 to 25 km. The northwestern border runs approximately from the mouth of the river Astura along the river and from its upper reaches to Cori in the Monti Lepini.