Actun Tunichil Muknal cave (usually referred to as “ATM”) is the doorway to Xibalba, the Maya underworld. This cave contains ceramics and human skeletons that have fused with the walls of the cave. The most famous of these remains is the Crystal Maiden, a skeleton of a young woman believed to have been a sacrifice. Her bones are now partially covered with the sparkling build-up of cave sediment.

This tour requires a good degree of physical fitness. First visitors will take a 45-minute hike through the tropical rainforest. The hike crosses three streams. There is a river that flows through the cave. To enter the cave, you’ll swim into the mouth of the cave. You’ll swim for 33 feet (10 m), in water that is 16.6 feet (5 m) deep. Eventually you’ll come to a rock shelf.

The ATM cave covers 3 miles (5 km). Some of the path through the cave is rocky, and some of the way is covered in pools of water. Most of the way through the cave you’ll either wade or swim. Along the way you’ll see stalactites or stalagmites.

After 600 feet (183 m), you’ll climb about 6.5 feet (2 – 3 m) to a ledge in the cave, above the water. This part of the cave is considered hallowed ground, where you will walk barefoot (but you are allowed to leave on your socks). After you ascend to the upper part of the cave, you’ll come across pottery of all sizes. Catalogued within the cave are over 1,400 artifacts - pottery, tools, and ceremonial items dating from 1 to 1000 AD - including over 200 vessels.

So far, archeologists have uncovered the remains of fourteen individuals, including 7 adults and 7 children. These are believed to be sacrificial victims. Experts believe that they were probably sacrificed in the hopes of appeasing the gods and bringing rain. Young victims, particularly females, were important for rituals to be pure and uncontaminated in the eyes of the gods. The Crystal Maiden is believed to have been one such sacrifice. To reach her final resting place, you must climb aluminum ladders mounted on a sheer rock face.