In the ancient past, the Seine spread throughout the entire bowl-shaped valley that now forms the Paris region, and at one time, it split into two arms. To the south, the slightly wider branch roughly followed the river’s present-day path. To the north, an arc of water swept across what is now the Right Bank, through Bastille, Menilmontant, parts of Belleville, and lower Montmartre. It reached all the way to the present-day locations of Chaillot and L’Alma, just across the river from the Eiffel Tower. When these two branches flooded, the whole basin filled to become a lake several miles wide. Little by little, the northern arm of the Seine dried up, and by 30,000 BCE, it had vanished completely, leaving more or less the Seine we know today. Large parts of the Right Bank remained wetlands for some time. The neighborhood called the Marais—which means “the swamp”—was once a marsh adjacent to the river. It is no wonder that when the Romans first invaded the area inhabited by the Parisii tribe in the first century BCE, they chose to construct their city on the less soggy Left Bank. No wonder either that they named the city Lutetia, likely derived from lutum, the Latin word for mud.
—Jeffrey H. Jackson, Paris Under Water