Krubera Cave in the republic of Georgia is currently the deepest known cave, at 7,208 feet. But Dark Star, with so many areas still to survey, is a prime candidate to take over the title.

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 ...