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This is one of the best things we've done so far!
From the town of San Ignacio, the van takes you for a twenty minute drive on the highway, then for another thirty minutes on an unpaved road, until you hit the trail. From there on a moderate forty five-minute hike takes you alongside river banks, through the jungle and a few times across the river until you get to the cave.
The hike inside the Aktun Tunichil Muknal cave (such a long name, no wonder all touring shops call it ATM) is quite demanding: swimming is required a few times; for the rest you're wading through water or clambering up and down craggy stones carved either by the water, by the spelunkers or maybe by the ancient Mayans who used the cave as a temple for performing their sacred rituals.
Because this is what it's all about - at the end of the long cave trail you reach the crux of this adventurous experience: the grand hall where the Mayans carried out their holy rituals and gave offerings to the gods. Many pots with well preserved colors are scattered around deep in the guts of the mountain, partially buried by calcite deposits, intentionally broken by the Mayans at the end of the ceremonials. Among the pots there are skeletal remains, considered by the archaeologists, with reasonable certainty, as victims of human sacrifices. Perhaps sometimes the gods were not satisfied with the pottery...
It is a wonder that the cave is still open to visitors, although some of the rooms used by the Mayans are only accessible to the archaeologists. A few of the pots have been broken by careless tourists in the past, so the tour agencies have invented a trick: as soon as you leave the water trail and enter the dry area, they ask you to remove your shoes and walk in your socks (they tell you about this in advance). It seems that when they are walking in their socks only, tourists are more prone to listen to their guide and only step where they are allowed to. There are no elevated, enclosed alleyways (like there would be, if this place were in the US) so the cave looks eerily natural and caution is necessary when you walk around. They don't make you sign any liability waiver... Thank god, there still are some places in this world that haven't been taken over by lawyers.