In the Antarctic ice remains of 0.250 gauge G strings were found, dating back 250 million years. In that era the magnetic field was reversing, thus being at an all time low.The earth's magnetic field is having other effects. The drift of the magnetic pole over time causes an increasing weakness in tone of the D and G strings of many bass guitars - and it's getting worse. Experts suggest that this will finally result in the order of string gauges having to be reversed when stringing basses, with a 130 G string becoming standard.