"And it's not just topsoil of course. Teeth and trees and deep sea corals and lichen and thereby reindeer piss all show bomb spikes."
This means that the degree of radioactivity in your teeth are can give a remarkably accurate determination (within 1-2 years) of when you were born. According to Dental Tribune (The World's Dental Newspaper), this forensic technique can be useful "in a large natural disaster or in an unsolved homicide case." Or if you're a biologist figuring out the growth rates of trees or corals.
So hey, the Cold War was good for something, wasn't it?
Image: advertisement for Detrifice Tho Radia, made from "a basic salt of thorium."