If a country fires an airborne nuclear missile, the source of the attack is obvious. But what about the more fluid threat that hangs over the 21st century—terrorists sneaking a nuclear device into a city and setting it off? In a study in the Proceedings of the National Academy of Sciences this week, researchers suggest that even in the charred aftermath of a nuclear explosion, there could be evidence left behind that helps to identify the source of the bomb. Physicist Albert Fahey and company went back to the beginning of the atomic age, to the United States' first atomic bomb test in New Mexico in July 1945. As that bomb test was called "Trinity," the glass left behind by the blast is called "trinitite." Fahey obtained some of that glass to show that all these years later, it still contained evidence of the bomb's makeup.