The Pontine Marshes, termed in Latin Pomptinus Ager by Titus Livius, Pomptina Palus (singular) and Pomptinae Paludes (plural) by Pliny the Elder, today the Agro Pontino in Italian, 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 (28 mi) from just east of Anzio to Terracina (ancient Tarracina), varying in distance inland between the Tyrrhenian Sea and the Volscian Mountains (the Monti Lepini in the north, the Monti Ausoni in the center, and the Monti Aurunci in the south) from 15 to 25 km (9.3 to 15.5 mi). 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.