Commentary Inf XV 4-12

The double simile compares the construction made by God to carry the 'water' of hell toward its final destination to the huge earthworks engineered in Flanders and in northern Italy to protect farmland and human habitations from flood. 'Carentana' probably refers (the exact reference is debated in the commentaries) to the mountains north of Padua from which the spring snows release their torrents.