Sistema Sac Actun (Mexico) – Located in the Yucatán Peninsula, it's one of the world’s longest underground rivers, connected to ancient Mayan cenotes. Lost River (USA – Indiana) – This river vanishes ...

Mesmerising as these tidbits are, the cave evokes wonder all by itself thanks to its limestone cavern and ... exploration as we muse over where in the Philippines we'd like to island hop along ...