The marsh was an extensive alluvial plain at about sea level (some above, some below) created by the failure of the streams draining the mountains to find clearly defined outlets to the sea through the barrier dunes. Above sea level, it was a forested swamp; below, it was mud flats and pools. Sparsely inhabited throughout much of their history, the Pontine Marshes were the subject of extensive land reclamation work performed periodically. The tribe of the Volsci began with minor draining projects in the vicinity of Tarracina in connection with their occupation of it in the pre-Roman period.