But the trees on the side of the road, instead of being vertical, lean down the hill. This makes the downward side of the hill seem to be the up side of the hill. The grade is so slight that it throws off your balance, so you become con- fused. After being in the room for a minute, your natural sense of balance tries to correct itself. Then, you try to roll a ball down some parallel bars, but the ball rolls up instead.
Walls are vertical.
So these mystery spots are usually on broad, empty plains with a slight slope. The slope isn't noticable, and rule 1 is applied by the brain to get a wrong answer for the horizontal. Hence any slight lessening of the slope looks like a slight upward rise on a flat plain, even though it is actually still downhill. Sometimes locals exploit rule 2 by putting up buildings that conform to the visual horizontal instead of the actual one. This reinforces the illusion quite strongly.