Actun Tunichil Muknal (ATM) is a cave in Belize, near San Ignacio, Cayo District, notable as a Maya archaeological site that includes skeletons, ceramics, and stoneware. There are several areas of skeletal remains in the main chamber. The best-known is "The Crystal Maiden", the skeleton of a teenage girl, possibly a sacrifice victim, whose bones have been calcified to a sparkling, crystallized appearance.