In most places, 6000 years would be plenty of time for cotton fabric to decay and disappear. Not in northern Peru. Thanks to the region’s arid climate, several decorative textiles survived for thousands of years until archaeologists dug them up in the 1940s. They were so well preserved that researchers can still see their colorful blue dyes. Radiocarbon dates revealed that the textiles ranged from 6200 to 1500 years old. But a mystery remained: What had these ancient cultures used to dye the fabrics blue? In a new study, researchers picked up signals of the chemical compounds indigotin and indirubin, the key components of indigo dye, they report today in Science Advances. Indigo was one of the most prized dyes in the ancient world and was used in places ranging from China to Egypt to South America. Today, indigo is still used to create the blue of your blue jeans. Previously, the earliest known use of indigo was 4400 years ago in Egypt. This new evidence pushes that date back by more than 1500 years and points to Peru as the place where people first learned to dye their fabrics a vibrant shade of blue.