A bar of gold unearthed in a park in Mexico City tells a remarkable story of colonial clashes, thieving conquistadors, and some very angry Aztecs.  

Archaeologists from Mexico's National Institute of Anthropology and History (INAH) recently revealed that this gold bar was one of the many Aztec treasures looted by Hernán Cortés and the Spanish conquistadors 500 years ago. Their analysis suggests that this ingot was perhaps lost during La Noche Trist, literally "The Sad Night,” when Cortés and his invading army was forced to hastily flee the Aztec capital at Tenochtitlan on the night of June 30, 1520.

The 1.93-kilograms (4.25 pounds) golden ingot was first unearthed in 1981 during construction work around Alameda Central, a public park in downtown Mexico City. 

A recent X-ray fluorescence chemical analysis of the bar – just in time for the 500th anniversary of The Sad Night – revealed that it has a composition of 76.2 percent gold, 20.8 percent silver, and 3.0 percent copper. This is almost exactly the same composition of the gold recovered from the Templo Mayor Project in Tenochtitlan. Even more precisely, the bar is extremely similar to the gold bars found around a monolith to the goddess Tlaltecuhtli at the temple. 

Compared to other gold pieces taken from Mesoamerican sites, such as the Maya objects extracted from the Sacred Cenote of Chichen Itza, the gold recovered from Templo Mayor had low concentrations of copper, just like this golden ingot. 

The INAH team concluded that this golden bar most likely originated in Tenochtitlan. In all likelihood, the ingot was removed from the city during that fateful night in 1520 when the Spanish fled the Aztec capital. 

“The Night of Sadness” was an especially bloody chapter in the story of the Spanish conquest of Mexico. Dr Leonardo López Luján, director of the Templo Mayor Project, explained in a statement the new evidence serves as a “key piece in the puzzle” of this historical event.

Cortés' expedition arrived at Tenochtitlan, the proud epicenter of the Aztec civilization, in November 1519. Amid a huge amount of tension and turmoil, the unwelcome guests pushed their luck too far on May 22, 1520, when they executed a number of prominent Aztec elites. Understandably disgruntled by the massacre, the Aztec people were enraged towards the brink of revolt. Within just a few weeks, according to Spanish accounts, they turned on the Aztec emperor Moctezuma and killed him. Tempers continued to boil over and a frenzied battle was sparked between the Aztecs and Cortés’ army, forcing them to flee the capital. 

The conquistadors escaped with as much looted Aztec treasure as they could hold. However, in the midst of the panic, it appears they dropped one of the prized gold bars. 

Of course, as we now know, the resistance from the Aztecs was short-lived. Within a year, Cortes' army was able to push back and started to siege the city of Tenochtitlan, marking the beginning of the grande finale in the Spanish conquest of the Aztec Empire.