The Actun Tunichil Maknal Cave (Cave of the Stone Sepulcher or ATM cave) can be found in the Tapir Mountain Reserve, about a 45-minute bumpy Jeep ride east of San Ignacio, Belize. It's a protected area, so you will need to join one of a few licensed tours to go there.
Once in the rain forest, its a fairly easy 45-minute hike in which you walk through a few shallow river crossings. Then you put a helmet on, swim into the cave (water is usually over one's head) and then hike, crawl, swim, and climb ladders about 1.5 kilometers into the "underworld" where the Mayas believed the gods lived. Toward the end of the cave is "The Cathedral," where 14 skeletons of sacrificed Mayas have rested for over 1,000 years. The most intact skeleton, "The Crystal Maiden," is calcified, giving it a sparkling, surreal appearance. Numerous pottery fragments also litter the floor.
Bats, freshwater crabs, crayfish, catfish, scorpions, and predatory spiders have made the river and cave their home. We only saw the bats and one spider. Otters have even been seen in the cave.
I liked that there were no ropes or walls separating visitors from the skeletons and pottery, but this has resulted in one tourist stepping on and crushing a skull. Please be careful.