One of my favorite research topics in Chicago History is the Fool Killer Submarine, which was found in the Chicago River in 1915, where it had been buried in river muck for decades. A couple of months ago PBS interviewed me for a story on it that aired last night. You can watch it here and see my Weird Chicago Blog entries about the thing (which constitute just about everything this is known about the thing) here.
Speaking of mysterious history, here's a new Playground Jungle post about the history of saying "jinx" when two people say the same thing at the same time. The practice may only be 50 years old - but it might also be 500!