Dowsing or water witching is the art of using a pendulum, water witching rods or your psychic abilities to find underground sources of water. Dowsing is a very specific form of divination (divination means accessing information not available to the normal five senses) and has been practiced for centuries and is still used very successfully today.
Probably the easiest way to start water witching is to use a pendulum. Almost any lightweight suspended object can be used but many dowsers use crystal pendulums from choice. To find underground water with a pendulum here's a short note from the American Society of Dowsers.
"The dowser usually seeks flowing underground veins suitable for drilling and pumping. The veins can be large and deep or small and shallow. You can discriminate by focusing on a need, real or imaginary . Ask mentally, either successively or all at once, for a vein of good drinking water, less than twenty feet in depth, which will flow uninterrupted at the rate of 5 gallons a minute or more. Thus when you receive a reaction, you will know that it is not a pipe, a polluted vein or a source that might otherwise be too deep or seasonally unreliable for the purpose at hand. When the dowsing reaction takes place, mark the spot and approach it from the opposite direction. If the two points do not coincide, the mid-point between them should be the center of the vein."
A left-to-right pendulum swing indicates a "no" while a back-and-forth swing indicates a "yes." If you find that the opposite works for you - go with it!
An alternative means of dowsing is to use two wire rods approximately between 8 " and 15" in length which can freely swivel. One should be held in each hand about 12" apart. When you move over the location that contains the underground source the rods will swing together and cross.