There are many "gravity hills" throughout the world http://en.wikipedia.org/wiki/Gravity_hill
I just visited one here yesterday in Burkittsville, Maryland: http://www.roadsideamerica.com/tip/600 There I put my car in neutral, and my car rolled uphill. I also spilled water on the ground, and the water traveled uphill.
One explanation is that gravity hills are an illusion caused by the landscape and the horizon. But there is a video on Youtube of a car traveling on the gravity hill in Burkittsville, Maryland in the dark, where one cannot see the landscape or the horizon, only the road ahead of the car illuminated by the car's headlights. In the youtube video, the car really looks like it is traveling upward.
So how does it work?
The only non-magical explanation that I can think of is that it is an optical illusion. I am looking for a detailed answer as to how such an optical illusion works.
Answer
The answer is in your Wikipedia link:
The slope of gravity hills is an optical illusion, although sites are often accompanied by claims that magnetic or even supernatural forces are at work.
The most important factor contributing to the illusion is a completely or mostly obstructed horizon; without a horizon, judging the slope of a surface is difficult as a reliable reference is missing. Objects one would normally assume to be more or less perpendicular to the ground (such as trees) may actually be leaning, offsetting the visual reference. The illusion is similar to the well-known Ames room, in which balls can also appear to roll against gravity.
In other words, anything that prevents you from seeing the horizon (like darkness, or surrounding hills) will allow your eyes to be tricked by the shape of the roadway.
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