People who lived in the Arabian Peninsula thousands of years ago went underground when they wanted to beat the heat. Possibly stopping there as they traveled between oases and pastures, they ducked into vast subterranean tunnels where molten lava had once flowed millions of years earlier, according to a recent study, CNN reports.

Beginning in the Stone Age, Neolithic herders descended into and occupied these vast tunnels, known as lava tubes, archaeologists have discovered. Cooler air underground would have provided a welcome respite from the sun and wind, and for thousands of years, humans sheltered with their livestock in the tunnels. The herders left behind objects and even carved pictures on the rocky walls, researchers reported in the journal PLOS One.

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

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

This discovery highlights the significance of Umm Jirsan and other tunnels for understanding human dispersal in the region, said Guillaume Charloux, an archaeologist with the French National Centre for Scientific Research.