Pontine Marshes, a low marshy plain in the S. part of the Campagna of Rome, extending 28 m. along the Mediterranean coast from Cisterna to Terracina. Its breadth varies from 4 to 11 m. These marshes are formed by the stagnation of the waters of the numerous streams which flow down the Volscian mountains or take their rise in springs at their foot, and are prevented from finding an outlet to the sea by the low level of the plain and the accumulation of sand upon the coast. The name of the marshes was derived from Suessa Pometia, a Volscian town on their borders, which about 500 B. C. disappeared from history, and the position of which is not now known. Various attempts were made by the Romans to drain the marshes. As early as 312 B. C. the Appian way was carried through this district, and with it a canal from Appii Forum to Terracina. In 160 B. C. a part of them was drained with apparent success by the consul Cornelius Cethegus; but the tract soon reverted to its original condition. Its drainage was again projected by Julius Cassar, and Augustus made some temporary improvements. Trajan restored the Appian way. During the wars preceding the downfall of the Roman empire the marshes were neglected.

Boniface VIII. drained the district about Sez-ze and Sermoneta by a large canal; and in 1417 Martin V. had a canal, called the Rio Martino, dug to within a mile of the sea, but the project was abandoned at his death. The district was granted by Leo X. to the Medici, upon the condition of their draining it, and it remained in their hands 69 years, during which time scarcely anything was done toward its improvement. A large canal, called the Fiume Sisto, was dug during the reign of Sixtus V.; but after his death in 1590 the dams gave way, and the country was again flooded. Pius VI. directed his attention to draining the marshes, and from 1777 to 1796 he expended $2,000,000 in the work. A part of the Appian way was restored, the road was continued across the marshes, and the canal attributed to Augustus was again opened under the name of Linea Pia. It has been found almost impossible, however, to reclaim the waste land, although there is a small portion under cultivation, and large pastures abound, where horses, cattle, and buffaloes graze.