Many tourists visiting the northern Italian city of Venice don't realise the city's very foundations are threatened by the water it's been famous for.

Environmental scientists say Venice has been slowly sinking for centuries.

Canals, gondolas and vaporetti, or water taxis. They are the familiar landmarks of Venice.

But the city's existence has been precarious because of its position. It's built on water, and it is also threatened by water on several fronts.

Jane Da Mosto, Environmental Scientist, said, "In a way Venice has always been in peril. Ever since the city was founded it's been in this delicate state of not exactly equilibrium but balance between land and water."

Scientists say the whole of northeast Italy where Venice is, is tipping gradually downwards. And Venice has been going down by about eight centimeters a century.

The level of the Adriatic Sea is rising. High tides are becoming more frequent, flooding into the famed St. Mark's Square. The city has had to set up raised walkways.