Barton Creek Cave (tours around US$60, including Bz$20 entry fee; you must be accompanied by a licensed guide to enter) is also accessible only by river, though this time by canoe. Framed by jungle, the cave’s entrance is at the far side of a pool, and inside the river is navigable for about 1600m before ending in a gallery blocked by a huge rockfall. If it’s been raining, a subterranean waterfall cascades over the rocks – a truly unforgettable sight. The clear, slow-moving river fills most of the cave width, though the roof soars 100m above you in places. Several Maya burial sites and pottery vessels line the banks, the most awe-inspiring indicated by a skull set in a natural rock bridge used by the Maya to reach the sacred site.