For millennia, Chile's man-made Chinchorro mummies, the oldest in the world, have remained in roughly the same stable condition—the result of a sophisticated mummification process that dates back 7,000 years. Now, they're turning to "black ooze," LiveScience reports. Researchers say that over the last decade, some of the 120 mummies at the University of Tarapaca museum in Arica have begun to decompose as rising humidity levels cause bacteria to thrive on the mummies' skin. The microorganisms "chew these guys right up," causing them "to go black and become gelatinous" when the moisture and temperature is just right, says Harvard's Ralph Mitchell, an expert in microbiology. Arica has long been arid, but fog off the Pacific has reached the area in the last 10 years.
"This kind of degradation has never been studied before," Mitchell tells the Harvard Gazette. To come up with a solution, Mitchell and his team tested pig skin as well as the mummies' skin in different levels of humidity. Skin began to disintegrate after just 21 days in high humidity. Researchers say the mummies should be safe when stored in a room with humidity at 40% to 60%—but the museum's mummies aren't the only ones at risk. Hundreds more are believed to be buried in northern Chile, which boasts man-made mummies 2,000 years older than those in Egypt. "You have these bodies out there and you're asking the question: How do I stop them from decomposing?" Mitchell says, per CNN. "It's almost a forensic problem." (More recent mummified corpses were found atop a mountain in Mexico.)