In this city on water, gondoliers paddle boats along 150 narrow canals and under 409 ancient bridges. Built on 117 small islands, Venice is considered one of the most beautiful cities in Italy, and the world. But erosion, pollution and dredging are causing the city of canals to slowly sink into the Adriatic Sea. All over the world, people are working to save the city and its precious landmarks and art work. (GETTY IMAGES)