Erwin Bonsma's ray-traced 3D illusions and a fractal

Below you find a collection of images featuring impossible objects. They are reminiscent of drawings by the Dutch artist M.C. Escher and have been inspired by those. The images below have all bar one been generated using ray-tracing; a process that renders 2D images from 3D scene descriptions.

No post-processing has been applied to generate these illusions. Each image is derived from a single virtual 3D scene. I encourage you to zoom into the details of the images: the textures, the shadows, the lighting, the reflections, etc. Can you work out how the illusion has been realized for each scene?