It's to do with certain geological conditions, such as water flowing beneath the ground, creating a field that can act like a tape-recorder. Certain people have the mental equipment to allow these recordings to be played back to them.
There's also some ideas that extreme emotion or trauma can imprint itself upon a location and be sensed by people - as recounted by many folks when they say they could feel the 'death' or 'happiness' at a certain location.
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