This is a terrific piece of art. Whoever has created it is able to achieve an excellent illusion. There is not much to explain and we believe you have already found out the illusion in this picture. If you look at the right most, you can find a ship. With perspective the ship size keeps on decreasing as you move your eyes towards the left. But as you move left, you can see that the sky is diffusing to form a bridge. The position of clouds and the ships is responsible to create this diffusion. It has been done with so much finesse that you won't even recognize when it swiftly diffuse to form the bridge. An amazing art hands down.