During a parched summer almost 2,000 years ago, people living in what is now western New Mexico crawled into the cold, dark belly of a volcanically formed cave to melt the frozen water at its heart. The ice preserved in these naturally cool formations might have helped Ancestral Puebloans in the region persevere through five such drought events over the course of 800 years, a new study suggests.

New analysis of charcoal particles from around A.D. 150 provides the earliest dated evidence that Ancestral Puebloans used fire to melt ice trapped deep in lava tubes when liquid water was scarce, researchers reported November 18 in Scientific Reports.The findings are evidence that these ancient people went to remarkable lengths to survive in an often hostile environment.

“This study demonstrates the ingenuity of Indigenous people who used the area,” says Barbara Mills, an anthropological archeologist at the University of Arizona in Tucson who was not involved in the study. “It also shows how knowledge about the trails, caves and harvesting practices was passed down over many centuries, even millennia.”

Ancestral Puebloans, forerunners of today’s Pueblo peoples and the builders of Mesa Verde’s famous cliff dwellings, survived in the arid southwestern United States for over 10,000 years. A key to that survival was finding creative ways to extract water from an unforgiving environment.

In April 2017, a team led by paleoclimatologist Bogdan Onac of the University of South Florida in Tampa traveled to El Malpais National Monument in New Mexico to collect ice cores from the park’s frigid lava tubes in the hopes of extracting ancient climate data. Lava tubes are an empty space left behind by flowing lava, a relic of the area’s volcanically active past. Far removed from their fiery beginnings, the caves retain a constant temperature around 0° Celsius (32° Fahrenheit) that can preserve accumulating ice — and anything trapped inside the ice — for hundreds of years. The cylindrical shape of the tubes causes cooler, denser air to sink toward the ground and push hotter, lighter air up and out.

Onac and his team initially planned only to extract paleoclimate data from the ice, but they found much more when they reached Cave 29. The interior of the 171-meter-long lava tube was covered with charcoal deposits concentrated around what was once a roughly 1,000-square-meter block of ice.

The team recovered a 59-centimeter-long core sample from what remains of the ice block, and noticed five distinct black bands that broke up its length. The presence of charcoal suggested fire, and the presence of fire deep in an icy cavern suggested human activity. Even more exciting, the charcoal’s position in the ice acted as a time capsule that allowed the researchers to date the periods of human activity. “When we got the core out and we saw the charcoal, obviously we were just jumping all over because that meant that we would have a chronology,” Onac says.

They melted the core down and radiocarbon-dated the charcoal pieces inside. Those dates — ranging from around A.D. 150 to A.D. 950 — corresponded to drought events recorded in tree rings in the surrounding area (SN: 6/1/20). The five charcoal bands’ chronological alignment with drought events suggests that hunters and travelers kept track of accessible water for survival and ceremonial practices over hundreds of years, the researchers say.

“The correlation of the radiocarbon dates with periods of drought is remarkable,” Mills says.

Researchers had previously suspected that Ancestral Puebloans once exploited the area’s lava tubes for freshwater. Ancient road networks crisscross the lava flows’ treacherous terrain, and pottery pieces and charcoal have been found in and around cave entrances. But evidence until now has been largely circumstantial.

A charcoal-coated shard of pottery, dated to A.D. 1097, found sitting on the ice block provided further evidence of human activity within the cave. The pottery piece’s recent emergence from the melting ice block was both exciting and concerning, Onac says, as it illustrates how fast the ice is melting as the climate warms.

Photographic evidence suggests that about 30 centimeters of ice has melted off the top of the block since the 1980s, which Onac estimates could represent hundreds of years of lost data. “We need to move fast because it’s melting pretty fast,” Onac says.