Actun Tunichil Muknal — usually shortened to ATM Cave — is the most extraordinary thing you can do in Belize and consistently ranks among the world's great cave experiences. The day begins with an hour-long jungle hike across three river crossings, then a swim through the cave's keyhole entrance and a kilometre of wading and scrambling through a flooded river passage. The reward is an upper dry chamber — a Classic-period Maya ceremonial site — where 14 calcified human skeletons, dozens of intact ceramics, and stone altars sit untouched in the chambers where they were placed more than a thousand years ago.
Visits are by licensed guide only — limited operators, capped numbers, no cameras allowed since a tourist dropped one on a skull in 2012. A trip with one of the better guides (Pacz Tours and Mayawalk are the long-running operators) runs about US$110 and takes most of a day. Children under 12 are not permitted; basic swimming is required. Book at least 24 hours ahead in high season.