In this image it’s hard to see the forest for the tree in the foreground. Our brain is being given all the right clues to not see what we are looking at.  

For starters, the image is in focus from front to back. A camera can accomplish this feat, but we don’t see things that way. Our visual system is so efficient at focusing on where we send our attention that the illusion of continuous focus is nearly perfect.

With the same efficiency, our brain assembles an image based on our own history and visual cues. We know that the trees behind the one in front are about the same size and that those in the distance appear to be smaller. This perspective allows us to create a usable sense of depth perception.

That same perspective creates a tree where there is none. The treasure of this illusion is found in the trunk of the tree in the foreground. Move your eye slowly from left to right across the trunk and you can see that the first tree in the background is really a shadow falling across the trunk of the tree in the foreground. 

Stinson Beach, California