The Tamarack Mines Mystery

...The essence of the story, as usually told, goes something like this. In the fall of 1901 J.B. Watson, Chief Engineer at the Tamarack copper mine (S. of Calumet, Mich.) suspended 4250 foot long plumb lines down mineshafts. Measurements showed that the plumb lines were farther apart at the bottom than at the top, contrary to expectations. Thus arose one of the long-standing mysteries of science. [continued]