Dowsers claim that they possess a natural sensitivity to alleged earth magnetism, water "radiations," or some other natural phenomenon. They believe their dowsing stick or other device (often an occult pendulum) somehow "focuses" or otherwise identifies this energy so that one is able to find water or other substances or things that one is seeking -- including oil, treasure, and lost persons or objects.
[...] 1. Can one dowser find the same spot twice? and 2. Can two dowsers can find the same spot once? I told them about this and their response was, ''We don't need to do the experiment because we know dowsing works. It's been around since the Middle Ages and the historical tradition validates its truthfulness.''
I challenge all the dowsers in a similar way. Since 94 percent of the Earth's surface has water within drillable distance my challenge is to find a dry spot! They don't want to do it. Why? Because they only have a six percent chance of success. Dowsing is an idiomotor reaction that is very deceptive. It is an unconscious motion that you cannot detect and it looks for all the world like some mysterious force.
Source: Randi at CalTech: A Report From The Paranormal Trenches, by James Randi