It seems that almost every state in the union has somewhere a "Mystery Spot" or "Magnetic Vortex" or "Spook Hill" or "Antigravity Cabin," or the like. What it is, is a tourist trap, and what it usually consists of is an old wood-frame house, usually in great apparent disrepair, and somewhere outside a plank of wood placed along the ground. What do you do there? Well, upon entering the house, you find yourself tilted at an incredible angle, seemingly nearly 40 degrees. Throwing a ball or pouring a liquid or deploying a plumb bob reveals the same inexplicable slant.

It's fun to search on the Internet and find "explanations" for this phenomenon. Although the actual explanation is quite obvious, the Internet (as it is on any topic, alas) is full of insensate gibberish about "gravitational anomalies," "magnetic vortices," "gravio-magnetic field warps," "wandering magnetic poles," "concealed mountains of lead," and even stranger scenarios, including hidden crashed alien spaceships whose engines are still operating. The actual and obvious explanation is no more than a structure that is built with an extreme tilt, and a well-concealed real horizon line. Creative use of hillsides is also common in these attractions. A related phenomenon is the "Magnetic Hill" or "Spook Hill" where automobiles appear to roll uphill. For a more detailed discussion of Mystery Spots, click here. A good history of such tourist traps is found here, with many nice photos. My favorite illusion is not the interior of the house, which admits of almost instant explanation, but the usual "plank of mystery" outside the house, as seen below.

I'll leave this second Mystery Spot illusion for you to figure out. Just remember such attractions are often built on a hill, and that there are no visible true horizon cues. It is just as easy to eliminate or conceal visible true depth cues. Remember the Ponzo illusion? Well, this is NOT an example of it! Hint: notice the concrete blocks under the plank, and the tilt of the mystery cabin in the background. The plank appears to be at right angles to the line of sight of the camera, but a man at one end of the plank appears much larger in size than the same man moved to the other end.  The illusion is slightly spoiled by the fairly obvious rotation of either man seen in the photos at left, when he is standing on the left end of the plank.  But it's still a pretty convincing and surprising illusion! If you can't figure it out, here is James Randi's very clear explanation with demonstrations!