radioactive material in ocean left over from a supernova

http://www.symmetrymagazine.org/article/august-2015/all-about-supernovae
"In 1998, physicists did analysis on debris from the bottom of the ocean and found radioactive iron. Because the radioactive iron, which should have decayed after a few million years, had not yet decayed, they determined that a nearby supernova must have dumped the material on our planet. They were able to date the radioactive material to nearly 3 million years ago."