In the early 1990s, several experiments were performed which hinted at the existence of a fifth fundamental force. The Australian mine experiment performed by Frank Stacey is discussed in Will (1990), as is Ephreim Fischbach's reanalysis of the Eötvös experiment. Fischbach concluded a small repulsive force was indeed present (assuming the technique to be correct, the required force was later shown to be attractive) over a range of 100-1000 meters which was different for different substances. Also discussed is the Greenland ice boring experiments of Mark E. Ander. Will concluded that, if present, a fifth and sixth fundamental force are constrained to be 1,000-100,000 times weaker than gravity over the range 1-1000 meters.
References
Will, C. M. "Twilight Time for the Fifth Force." Sky and Telescope, Nov. 1990.
