An international team of scientists has found evidence, in the form of radioactive iron-60 in sediment and crust samples taken from the Pacific, Atlantic, and Indian Oceans, which is indicative of a series of massive supernova explosions that showered the Earth with radioactive debris between 3.2 and 1.7 million years ago. The scientists believe the series of supernovae were less than 300 light years away -- Close enough to be visible during the day and comparable to the brightness of the Moon at night. Although Earth would have been exposed to an increased cosmic ray bombardment, the radiation would have been too weak to cause direct biological damage or trigger mass extinctions.