This is part of the Actun Tunichil Muknal Cave (ATM) in the jungles of Belize.
The ATM cave is a 45 minute drive from San Ignacio, followed by a 45 minute hike. The hike includes three stream crossings and passes through the 6,700 acre Tapir Mountain Nature Reserve. You have to swim into the cave with full gear and then wade & climb on wet rocks through the cave.
As you reach the main chambers of the cave, you are required to remove your shoes and walk along specific ridges in order to not disturb the site or it's artifacts.
As a Maya archaeological site, ATM is notable for its skeletons, ceramics, and stoneware. The most famous of the human remains is known as "The Crystal Maiden". It is the skeleton of a teenage girl, probably a sacrifice victim, the bones of which have been completely covered by the natural processes of the cave, leaving them with a sparkling appearance. There are several such skeletons in the Main Chamber.
The ceramics at the site are significant partially because they are marked with "kill holes", which indicates they were used for ceremonial purposes.
This was an incredible experience - I'm actually really surprised they even let people in these caves - so close to these valuable artifacts!