ON AUGUST 27, 1883, a volcanic explosion smashed the Indonesian island of Krakatau with the force of a 100-150 megaton fusion bomb. The sound from this blast, the largest in recorded history, could be heard 4600 kilometers away. The explosion formed an undersea crater seven kilometers wide and 270 meters deep, swallowing most of the island. The rest of Krakatau was incinerated by a layer of pumice ash forty meters thick at several hundred degrees centigrade, hot enough perhaps to melt lead. The island was completely sterilized.