Back in 2013, Timothy Koeth, an associate research professor at the University of Maryland, received a rather extraordinary birthday gift: a little cloth lunch pouch containing a small object wrapped in brown paper towels. As Koeth peeled back the layers, his eyes grew wide with astonishment. He immediately asked, "Where did you get that?" Inside he found a heavy metal cube and a crumpled message, a provocative note wrapped around a stone that came crashing through the window of history. It read, "Taken from Germany, from the nuclear reactor Hitler tried to build. Gift of Ninninger." Koeth's friend grinned, picked...