The Mayans believed that a cave was the pathway to the underworld, a connection to the Gods. The cave at Barton Creek is a wet cave (water is dripping from the roof and a creek flows trough it) and was therefore thought to belonged to the rain God, Chac. So to make him happy and get a good harvest, they sacrificed what Mayan Gods apparently wanted: humans. Today, the tour into the cave is done by canoe with one of the park rangers acting as a guide. Bits and pieces of Mayan artefacts (including a skull) have been laid out to be viewed from the canoe, but you don't get on land to see the actual sacrificing place where the bones are. If Mayan bones are more your thing, you might want to try the more expensive Actun Tunichil Muknal cave (ATM cave) further east which requires hiking, scrambling and swimming to get to.