Lazio comprises a land area of 17,236 km2 (6,655 sq mi) and it has borders with Tuscany, Umbria, and Marche to the north, Abruzzo and Molise to the east, Campania to the south, and the Tyrrhenian Sea to the west. The region is mainly flat and hilly, with small mountainous areas in the most eastern and southern districts. The coast of Lazio is mainly composed of sandy beaches, punctuated by the headlands of Circeo (541 m) and Gaeta (171 m). The Pontine Islands, which are part of Lazio, lie opposite the southern coast. Behind the coastal strip, to the north, lies the Maremma Laziale (the continuation of Tuscan Maremma), a costal plain interrupted at Civitavecchia by the Tolfa Mountains (616 m). The central section of the region is occupied by the Roman Campagna, a vast alluvial plain surrounding the city of Rome, with an area of approximately 2,100 km2 (811 sq mi). The southern districts are characterized by the flatlands of Agro Pontino, a once swampy and malarial area, that was reclaimed over the centuries. The Preapennines of Latium, marked by the Tiber valley and the Liri with the Sacco tributary, include on the right of the Tiber, three groups of mountains of volcanic origin: the Volsini, Cimini and Sabatini, whose largest former craters are occupied by the Bolsena, Vico and Bracciano lakes. To the south of the Tiber, other mountain groups form part of the Preapennines: the Alban Hills, also of volcanic origin, and the calcareous Lepini, Ausoni and Aurunci Mountains. The Apennines of Latium are a continuation of the Apennines of Abruzzo: the Reatini Mountains with Terminillo (2,213 m), Mounts Sabini, Prenestini, Simbruini and Ernici which continue east of the Liri into the Mainarde Mountains. The highest peak is Mount Gorzano (2,458 m) on the border with Abruzzo.