Deep in the caves of Southern Mexico, UNLV geoscientist Matthew Lachniet hopes to discover why some of North America’s most prosperous early civilizations died out.

The evidence he’s looking for isn’t among some hidden treasure that Indiana Jones might pursue or depicted in ancient cave paintings. Lachniet is looking for his evidence in stalagmites, the conical stacks of mineral deposits rising from cave floors.

These common cave formations act as ancient rain gauges that have recorded long-term climate change. Lachniet and an international team of researchers have used them to establish 2,400 years of the climate history of southwestern Mexico; this history, coupled with archaeological evidence, links the rise and fall of ancient Mesoamerican civilizations to changing rainfall.

The team’s findings were published online recently in the journal Geology.

Stalagmites form below stalactites, which form on cave ceilings. When tiny drops of water and calcite minerals drop off stalactites and accumulate on the cave floor over thousands of year, stalagmites develop – and much like the rings of a tree, they accurately record the rainfall history of an area.

Little is known about what contributed to the growth and downfall of the ancient Mesoamerican city of Teotihuacan, though historical evidence suggests periods of above average rainfall followed by extreme drought might have played a role.

To find the answers, Lachniet and his team collected and analyzed a stalagmite from Juxtlahuaca Cave in the Mexican state of Guerrero. The cave is located in the core region affected by the North American Monsoon, a climate phenomenon primarily responsible for rain in most of Mexico and parts of Arizona, New Mexico, and Southern Nevada.

Researchers first verified the rainfall record of the stalagmite by comparing deposits from the tip of the stalagmite with known rainfall amounts from the more recent past. Water samples were also collected deep within the caves to calibrate chemical variations in the stalagmites and unravel the climate history hidden within.

“Mexico may seem far removed from Southern Nevada, but the two regions are in fact linked by climate processes in the Pacific Ocean. Our new record shows that dry conditions, likely linked to El Niño processes, recurred frequently over time,” Lachniet says. “The point to be made is that civilization runs on water. Take away a water supply, and the civilization may fail.”

Lachniet and his colleagues correlated the region’s cultural milestones with measured rainfall amounts. Above average rainfall between the first and third centuries, for example, coincided with the rise of the largest early Mesoamerican city of Teotihuacan. At its peak, more than 125,000 people lived in the highly developed city.

Conversely, a 500-year drying trend, including a drought of more than 150 years, coincided with rapid population decline in Teotihuacan around 550 CE. The drought likely impacted dry-land agriculture practices in the semi-arid Mexican Highlands.

Researchers argue that another drought, this one from 690-860 CE, made it difficult for the basin area to sustain large urban areas. Archaeological evidence from this dry period also includes smashed “Storm God” artifacts, which may have signified abandonment of the civilization’s rain god.

“We can’t say with certainty that other social factors weren’t drivers of the cultural change, but we now have welldated and robust climate information to compare,” Lachniet says.

The study was published in the journal Geology and was supported by grants from the National Science Foundation and the National Geographic Society. Partnering with Lachniet were Juan Pablo Bernal of Mexico’s Centro de Geociencias in Juriquilla; Yemane Asmerom and Victor Polyak of the University of New Mexico; and Dolores Piperno of the Smithsonian National Museum of Natural History.