In the Barbara Kingsolver novel "The Lacuna," a small boy dives down into a body of water and discovers a lacuna: a missing section of earth. His caregiver tells him it's "not a cave exactly but an opening, like a mouth, that swallows things." One day, when the tide is at its lowest, the young boy swims through the lacuna and finds himself in a secret world full of broken coral and human bones. When the tide rises, it fills the lacuna and the boy is sucked out and, nearly drowning, he's spit into the open sea.
Underwater cave diving is equally mysterious, dangerous and compelling. Divers are required to obtain additional certification and must acquire specialty equipment in order to attempt it. From the popular cave dives at Peacock Springs State Park, Fla., to the tricky depths of Blue Hole in Dehab, Egypt (known as the "diver's cemetery"), underwater cave diving is one of the most difficult, dangerous sports there is. The stalwart souls brave enough to attempt it are rewarded with lamp lit views of the world's most secret sanctums.
For explorers who prefer to delve the depths without worrying about running out of oxygen or tangling their hoses on stalactites, we explore submarine tours in the next section.