Layers of a 3D maze are shown from top to bottom. You can move horizontally within a layer if there is no wall between compartments. You can move from a compartment with white floor to the the compartment immediately below it and back. Find the shortest path from the red to the green point.
 M. Gardner, The Colossal Book of Short Puzzles and Problems (D. Richards, ed.), New York: W. W. Norton, 2006 pp. 280–281.