Actun Tunichil Muknal (ATM), also locally known as "Xibalba", is a cave in Belize, near San Ignacio, Cayo District, notable as a Maya archaeological site that includes skeletons, ceramics, and stoneware. One in particular is the famous "Monkey Pot", one of only four located across Central America. The most famous of the human remains is known as "The Crystal Maiden", the skeleton of a teenage girl, probably a sacrifice victim, whose bones have been completely calcified by the natural processes of the cave, giving it a sparkling, crystallized appearance. There are several areas of skeletal remains in the main chamber.

The ceramics at the site are significant partly because they are marked with "kill holes", which indicates they were used for ceremonial purposes. Many of the Mayan artifacts and remains are completely calcified to the cave floor. The Mayans also modified cave formations here, in some instances to create altars for the offerings, in others to create silhouettes of faces and animals, or to project a shadow image into the cave. The cave is extensively decorated with cave formations in the upper passages. Part of the cave extends over the border into Guatemala, where it ends.

Animal life in the cave includes a large population of bats, large freshwater crabs, crayfish, catfish and other tropical fish. Large invertebrates like Amblypygi and various predatory spiders also inhabit the cave. Agouti and otters may also use the cave. These and many other species are quite common in river caves of this size in Belize.