The Arno was a river flowing through Florence on Earth.
When the Leonardo da Vinci-hologram and Kathryn Janeway tested Da Vinci's glider for the first time in 2374, they started off a bridge and immediately fell into the Arno. Later, when the two prepared to take flight on an improved version of the glider on an alien planet, Leonardo da Vinci noted that "Unfortunately, the river Arno is quite absent." (VOY: "Concerning Flight")