Venice lagoon formed part of a network of inland waterways used for navigation in the northern Adriatic, an almost continuous system of lakes, river-mouths and canals, at least partly pre-Roman in origin. The fossae which cross the present lagoon are the continuation of a complex system of natural watercourses and artificial canals between Ravenna and Aquileia. Two Roman buildings discovered on the present San Felice canal could be interpreted as providing navigational assistance at points linking the sea and inland routes.