The former marsh is a low tract of mainly agricultural land created by draining and filling, separated from the sea by sand dunes. The area amounts to about 80,000 ha (800 km2; 310 sq mi). The Via Appia, a Roman military road constructed in 312 BC, crosses the inland side of the former marsh in a long, straight stretch flanked by trees. Before then, travelers had to use the Via Latina along the flanks of the mountains; Terracina could not be reached across the marsh.