"Unfortunately careful tests of dowsers have revealed absolutely no ability to find water or anything else by extra-sensory perception. Dowsing success stories can be explained by noting that wherever you dig you will find water. You just have to dig deep enough. It has also been suggested that dowsers may unconsciously use clues in the environment.
"James Randi has tested more than 100 dowsers (I don't know the actual count). He tells that only 2 tried to cheat. This suggests that dowsers are basically honest people.
"The Skeptical Inquirer has published a number of articles on dowsing. James Randi's "A Controlled Test of Dowsing" was in vol. 4, no. 1, pp. 16-20. Michael Martin's "A New Controlled Dowsing Experiment" was in vol. 8, pp. 138-140. Dick Smith's "Two Tests of Divining in Australia" was in vol. 6, no. 4, pp. 34-37. Randi's book Flim-Flam! has a section on dowsing. The main skeptical book about dowsing is Vogt, E.Z. and Hyman R. (1959, 2nd edition 1979) Water Witching USA. The University of Chicago Press. 260 pages. Available as a paperback."
http://www.arches.uga.edu/~skepticx/skep_2.html - no longer active page
"Randi (1979) conducted a test with four dowsers in Italy, using three buried pipes with running water. As stated by Chamberlin (1980) the test had several deficiencies. No meaningful statistical evaluation was possible, and the test contributed little knowledge to the scientific community."
"Map dowsing tests he conducted with people claiming such ability were unsuccessful (Tromp, 1968)."
"Whitton and Cook (1978) also conducted two experiments attempting to determine whether subjects could detect the presence of weak magnetic fields.... Only chance results were obtained."
"Gallay (in Roberts, 1953) reported on a short study conducted with Henry Gross by a group of electrical engineers familiar with Tromp’s work....subsequent trials did not obtain significant results."
"Map dowsing tests were conducted at the Parapsychology Laboratory with subjects as far away as Germany. No significant results were obtained."
"Another series of unsuccessful experiments was reported in Nature by R. A. Foulkes (1971). Experiments were organized by the British Army and Ministry of Defence to determine if buried mines could be located by either map or field dowsing; dowsing for water was also tested. Again only chance results were obtained."
[This is a very good website. It has a great deal
of information about the experiments - bfg]
"Testing has been sparse, however. For one thing, it is difficult to establish a "baseline against which a diviner's performance may be compared" (Zusne and Jones 1989: 108). In 1949, an experiment was conducted in Maine by the American Society for Psychical Research. Twenty-seven dowsers "failed completely to estimate either the depth or the amount of water to be found in a field free of surface clues to water, whereas a geologist and an engineer successfully predicted the depth at which water would be found in 16 sites in the same field...." (Zusne and Jones 1989: 108; reported in Vogt and Hyman: 1967)."