A pair of "hookah" divers -- essentially free divers with a hose and regulator that lead back up to a boat -- is silhouetted above the sink hole "Blue Hole."

There are several of these submerged open sinkholes among the islands of the Exumas. This one is about 100 feet across and almost perfectly circular. The top is in about 45 feet of water but the bottom is much deeper, well below 100 feet.

With better visibility this might have been a more compelling dive. But you could only barely see across the lip of the cavern, concealing an otherwise remarkable formation.