No. 33: May-Jun 1984
Measurements of G, the constant in Newton's Law of Gravitation, made in mines are always significantly higher than those made in surface laboratories. No one is quite sure why. It has been suggested that a lack of knowledge of the densities of surrounding rocks might account for this discrepancy. But Holding and Tuck report new measurements in Australian mines, where the densities are very well known. The G values are still high. Mine measurements of G differ in the fact that the masses employed can be much farther apart.
(Holding, S.C., and Tuck, G.J.; "A New Mind Determination of the Newtonian Gravitational Constant," Nature, 307:714, 1984.)