MEXICO CITY, MEXICO—According to a report in The Guardian, fluorescent X-ray chemical analysis conducted by scientists at the National Autonomous University of Mexico has revealed that a four-pound gold bar, unearthed in downtown Mexico City during a 1981 construction project, was cast between 1519 and 1520. At that time, historical records indicate that Spanish conquistador Hernán Cortés melted down gold objects taken from the Aztec treasury and formed them into gold bars for transport to Europe. The conquistadors are thought to have dropped this gold bar in what had been a canal in the Aztec capital city of Tenochtitlan while fleeing a battle that began after Moctezuma, the Aztec emperor, was assassinated. A year later, in 1521, Cortés returned and laid siege to and captured Tenochtitlan, whose residents had been weakened by smallpox. For more on Tenochtitlan at the time of Cortés' arrival, go to "Under Mexico City."