Outlines the difficulties of dating material, through carbon-14 methods, where the material is older than the calibrated values obtained by comparison with tree-ring dates due to errors introduced in carbon-14 dating by variation of carbon-14 in ancient atmospheres. The recent datings of coral through uranium-thorium techniques and the observed discrepancies of these results and those obtained by carbon-14 dating of this material have allowed calibration of the carbon-14 technique back to 30,000 years. The discrepancy is described as possibly being due to fluctuations in the strength of the earth's magnetic field, which allowed the entry of more high-energy particles from space. Thus more carbon-14 was produced during periods of low field strength.