Venice
There are several problems in Venice, but it is still the most popular tourist attraction in northeast Italy. Venice now has about as many beds for visitors as for inhabitants: about 49,000 each.  Every year the city sinks a few millimeters because the ground is made from mud. Eventually, the city might be completely underwater, but that would take decades. Because of this, the Italian government is building the MOSE Project, a state-of-the-art defense against the sea-water flooding, that will safely protect Venice indefinitely.