In late June of 1908, a fireball exploded above the remote Russian forests of Tunguska, Siberia, flattening more than 800 square miles of trees. Researchers think a meteor was responsible for the devastation, but neither its fragments nor any impact craters have been discovered. Astronomers have been left to guess whether the object was an asteroid or a comet, and figuring out what it was would allow better modeling of potential future calamities. Italian researchers now think they've found a smoking gun: The 164-foot-deep Lake Cheko, located just 5 miles northwest of the epicenter of destruction. "When we looked at...