Periodically in the prehistory of the earth, mass extinctions have wiped out masses of species. Courtillot (geophysics, U. of Paris, France) argues the case for believing that these mass extinctions were almost solely the result of volcanic activities. After comparing the other proposed cause of asteroid impact, he details the correlations between volcanic activity and extinctions and explains the likely results of heightened volcanic activity. He also cites the Chicxulub impact crater in the Yucatan as the strongest piece of countervailing evidence and calls into question some of the conclusions drawn from the crater.