A few years ago, we were fortunate to be able to visit the Actun Tunichil Muknal (Cave of the Crystal Sepulchre, a.k.a. ATM) cave in Belize. It’s a 3-mile long wet cave (i.e., there’s a stream flowing through it), and a pretty “tough ticket” in that only a few guides are permitted to take tours through it — and the number of groups passing through on any given day are strictly limited. It’s a physically demanding visit, too — you wind up climbing over and under boulders, swimming a significant part of the way, etc.
But the restrictions are all for good reason. The cave was used for sacrificial offerings by the ancient Maya, largely during the classic period (roughly 250 – 900 AD). As the classic period wound down and the local situation worsened, increasingly dear sacrificial offerings were made increasingly far into the cave. Tours extend as far as “The Crystal Maiden,” the calcified skeletal remains of a teenage girl sacrificed near the end of the classic — but she is only one of 14 individuals whose remains have been found in the cave, and less macabre offerings predominate anyway.
Sadly, in 2012 a tourist dropped their camera on one of the 1,000 year old skulls in the cave and fractured it — the skull was repaired to some degree, but as a result of that one tourist’s inattention, visitors are no longer allowed to bring cameras into the cave.