Regardless of what it might have been in previous times – farmland initially defended against the encroaching sea and then repeatedly submerged in the 4th and 5th Centuries, as some recent theories claim, or a vast zone continuously invaded by water, according to the traditional view – the fact remains that when the city of Venice started to take shape (in roughly the 7th Century), the lagoon was an extensive area lying between the sea and the mainland where the rivers flowing down into the sea from the "Po" Valley met, turning into canals before finally reaching the sea through the apertures in the outermost sandbanks.