Around 11,000 years ago, humans first set foot in the driest place on Earth.

The Atacama Desert straddles the Andes Mountains, reaching into parts of Chile, Peru, Bolivia and Argentina. Little rain falls on the desert — some spots haven’t received a single drop in recorded history.

But the people who arrived at the Atacama managed to turn it into a home. Some Atacameños, as they are known today, fished the Pacific. Others hunted game and herded livestock in the highlands. They mummified their dead, decorating them with ceremonial wigs before leaving them in the mountains.

Those mummies reveal a hidden threat in the Atacama. When scientists analyzed the hair in 7,000-year-old mummy wigs, they discovered high levels of arsenic. Through their lives, the Atacameños were gradually poisoned.

Arsenic can poison people today through exposure to pesticides and pollution. But arsenic is also naturally present in the water and soil in some parts of the world. The Atacama Desert, sitting on top of arsenic-rich volcanic rock, is one of them. The concentration of arsenic in Atacama drinking water can be 20 times higher than the level considered safe for human consumption.