Actun Tunichil Muknal, Belize

Actun Tunichil Muknal means the Cave of the Crystal Sepulchre, and locally it is just ATM. It lies near San Ignacio in the Cayo District of Belize, and it is a Maya archaeological site that was left more or less as its last visitors left it.

The main chamber holds skeletal remains in several places. The best known is the Crystal Maiden: the skeleton of what is believed to be a seventeen-year-old boy, possibly a sacrifice, whose bones have calcified over centuries until they sparkle.

The pottery tells you what the cave was for. Many pieces carry kill holes, deliberately made to release the spirits held inside them, which marks them as ceremonial rather than domestic. One vessel, the Monkey Pot, is one of only four of its kind found anywhere in Central America. A great deal of the material is now completely calcified to the cave floor, fused into the rock it was left on.

The Maya also worked the cave itself, modifying formations, in places to make altars for offerings. It has been suggested, though not shown, that some were shaped to throw shadowed silhouettes of faces and animals onto the walls.

The upper passages are heavily decorated, and the cave is thoroughly alive: a large bat population, big freshwater crabs, crayfish, catfish and other tropical fish, along with amblypygi and various predatory spiders.

Visiting means swimming in, wading and climbing, on a guided trip only. Cameras have been banned since a visitor dropped one on a skull.