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People who lived on the Arabian Peninsula thousands of years ago went underground when they wanted to beat the heat. Possibly stopping there while traveling between oases and grasslands, they dove into vast underground tunnels where molten lava flowed millions of years ago, according to a recent study.

At the start of the Stone Age, Neolithic shepherds descended and inhabited these huge tunnels, known as lava tubes, archaeologists have found. The cooler air underground would provide a welcome respite from the sun and wind, and for thousands of years people have sheltered their livestock in the tunnels. Herders left objects and even carved paintings on the rocky walls, the researchers reported April 17 in the journal PLOS One.

In the Harrat Khaybar lava field, about 78 miles (125 kilometers) north of Medina in Saudi Arabia, there is a tunnel system called Umm Jirsan, the longest in the region. Scientists have not yet confirmed the age of the lava that formed this system, but a 2007 study suggested it was about 3 million years old. Umm Jirsan stretches nearly 1 mile (1.5 kilometers) with passages that are up to 39 feet (12 meters) high and 148 feet (45 meters) wide.

Archaeologists at Umm Jirsan recently discovered animal bones dating from 400 years ago to more than 4,000 years ago, and human remains ranging from 150 years ago to about 6,000 years ago. The research team also found pieces of cloth, pieces of carved wood and dozens of stone tools – the first evidence that humans used the tunnels starting at least 7,000 years ago.

“We knew from earlier reports that the fossils were preserved at the site,” said lead study author Dr Matthew Stewart, a research fellow at the Australian Research Center for Human Evolution at Griffith University in Australia.

“However, we did not expect to find evidence of human activity in the form of rock art, stone artifacts, stone structures and pottery,” Stewart told CNN in an email. “People have used and inhabited these lava tubes for millennia. While most research in Arabia has focused on surface sites, subterranean settings such as Umm Jirsan offer enormous potential to fill some of the data gaps.

This discovery highlights the importance of Umm Jirsan and other tunnels in understanding the dispersal of people in the region, said Guillaume Charloux, an archaeologist at the French National Center for Scientific Research. In general, knowledge about the ancient climate and people in northwestern Arabia is limited, “especially during the transition phase between the Neolithic and the early 2nd millennium,” said Charlou, who studies ancient sites in Saudi Arabia but was not involved in the new research . .

At that time, local people settled around recently formed oases; the emergence of these desert refuges will shape human migration patterns in the region for millennia, he said by email. “The main contribution of this innovative and major research project seems to me to be that it brings to light the long-term use—probably an ephemeral occupation—of this type of cave that has remained unexplored, and their enormous potential, especially for understanding paleoenvironmental contexts.”

For nearly 15 years, Stewart and his colleagues have collected evidence of ancient human life in Arabia, mostly from sites around lacustrine deposits, Stewart said. Beginning about 400,000 years ago, recurring wet periods saturated the Arabian deserts with rainfall. During these phases of “Green Arabia,” lakes and ponds abounded and the landscape bloomed with lush vegetation, leading to waves of migrating people that spread across southwest Asia, Stewart and other researchers previously reported in the journal Nature.

But the last phase of Green Arabia was about 55,000 years ago, and the harsh desert environment was not conducive to archaeological evidence. While stone tools preserve well in dry deserts, bones and other organic materials are easily broken down and destroyed by erosion and extremes of heat and cold, leaving little for researchers to interpret, Stewart noted.

“To this end, in 2019 we decided to explore underground conditions where organic matter and sediments can be better preserved,” he said.

So scholars turned their attention to Umm Jirsan. The site was previously mapped by the Saudi Geological Survey and a 2009 report described it as a haven for wildlife such as foxes, wolves, birds and snakes. Bone caches in the tunnels included human skull fragments estimated at the time to be around 4,000 years old. But as of 2019, the tunnel system had not yet been closely studied by archaeologists, Stewart said.

“We were able to date the animal bones and sediments, which informs us that humans began inhabiting the cave 7,000 years ago and perhaps as early as 10,000 years ago,” Stewart said.

Compared to other places where people once lived, the amount of archaeological material at Umm Jirsan is “rather sparse,” suggesting that people visited the tunnels as temporary shelters rather than living there permanently, the study authors reported.

In another tunnel near Umm Jirsan, researchers discovered 16 panels of engraved rock art. The carvings looked like herd scenes, with people carrying tools, stick figures standing next to domestic animals such as dogs, cattle, goats and sheep. Other engravings showed animals with dramatically curved horns resembling those of an ibex; however, these horned animals may represent a different breed of domesticated goat, according to the study. The subjects of the carvings and their lacquer coating suggest that they date from a regional period known as the Chalcolithic (ca. 4500 to 3500 BC), which preceded the rise of the Bronze Age.

“Taken together, the archaeological finds at the site and in the surrounding landscape paint a picture of the repeated use of the Umm Jirsan lava tube over millennia,” Stewart said. The site, which is along a known migration route for Bronze Age herders, “may have served as a stopover, a place of refuge protected from the elements.”

This unprecedented evidence of human activity in ancient Arabian lava tubes sheds light on how humans adapted to live in arid landscapes, and further study of Umm Jirsan and other lava tubes promises to add even more detail, Stewart added.

“These sites have enormous potential to fill some of the natural and cultural record gaps that persist in the Arab archaeological record.”

Mindy Weisberger is a science writer and media producer whose work has been published in Live Science, Scientific American, and How It Works magazine.