Infra-sound is reasonably well known to produce effects at certain frequencies, some of them quite distressing (nausea, pain, etc). There was an experiment in France several decades ago to investigate these effects. They found that a frequency of 7Hz produced the most extreme effect on humans. There was even a suggestion that it could be used as a weapon. Some rooms can exhibit infra-sound resonances that can be stimulated by noises in the environment or by wind. People can feel ill or uneasy in such rooms (maybe halucinate too?).
Although most dowsers walk fairly gently over the ground, I suppose that it's just possible that they are producing seismic impulses with their feet and then somehow detecting the return "echos". That's pure speculation, though.
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