An ancient marvel of the sea is gracing Sydney Harbour. Belly down, it has glided past yachts and waterfront homes, a beast of inflated proportions, before coming to rest at a secret location.

Is it a bird? A plane? No, it's Alphie, a 15-metre inflatable turtle here to bring art to the people of Sydney.

Alphie the turtle is moored among yachts in a secret Sydney Harbour location. Credit:Don Arnold/Getty Images

The floating sculpture formally known as the "Alpha Turtle" is emblazoned with an abstract black and white depiction of coral reef structures and hopes to inspire Sydneysiders to think about reef health.

"I think everyone is going to relate to Alphie," his guardian and creator, artist BJ Price, said.