Staring at the periodic table of elements was pretty much all I did in science classes but I learned absolutely nothing from it other than Bromine and Barium (but that's more because of a Google search on Breaking Bad). Anyways! Elements on that periodic table aren't all created equal. Especially Astatine (At). That's pretty much the hardest thing to find anywhere.
The NPR says that Astatine is so rare that it's never even been seen directly because if you managed to actually get enough of it together, it would vaporize itself with its own radioactive heat. In fact, on the entire Earth, there is only one ounce of Astatine.
So what the hell? How did we even discover it? It took a pretty long time. When they were making the periodic table, no one had actually seen an atom with 85 protons (that's how much is packed in Astatine's nucleus) before so they just kind of assumed they would eventually run into it. They did. But that doesn't mean they do often!