(CNN) This stunning, one-of-a-kind UNESCO World Heritage Site is losing its battle with the sea.

The city built on 118 small islands is facing its worst flooding in half a century. Six-foot-high tide levels have engulfed 85% of its streets and buildings.

Venice's mayor says the city has suffered "grave damage."

He's calling on the world to help restore its architectural masterpieces, paintings and monuments -- and to help rebuild flooded homes and businesses.

Here's how you can help