Mexico holds many enchanting destinations south of the border, including this underground river where you can swim in a cave ...

Outside the cave, it’s a blistering ... the hall like galaxies of stars in a crystal clear night sky. Zhenya Tsurikhin climbs a free-hanging rope. Dark Star’s passages are deep underground ...

I glance at my guide, Brian Kakuk—one of the world's foremost cave divers. He appears unfazed ... structure of the inland blue holes—deep, dark, sheltered, with little tidal flow—specialized ...