Actun Tunichil Muknal (ATM) is one of the most popular and impressive Maya ceremonial caves ever found.
ATM was in use during the early classic period, which dates to around 300-600AD. The caving part of the tour, takes you through a subterranean river, over (and under) more than a few rocks and into the main chamber.

The ATM Cave is a wet cave – you swim into the mouth and then wade through ankle to chest high water before climbing to a intricate passageways of dry chambers which lead to the cathedral-like main hall, a towering chamber 350 metres in length and 50mteres wide and filled with artefacts and sacrificial remains.
After this another climb takes you to the stone chamber where the Crystal Maiden lies, this is the skeleton of a teenage girl who was sacrificed and left in the cave. Over thousands of years her remains have been covered by a layer of diamond like limestone. The Mayas sacrificed humans in the hopes of appeasing the gods and bringing fertility to the land.