Tourists and hard-core adventurers are attracted to underwater caves around the world, some hundreds of feet deep. Free divers sink into the darkness without supplemental oxygen for record-breaking dives. Above, the Blue Hole in Belize Barrier Reef.
A scuba diver explores the Blue Hole Cave in Micronesia, Palau.
A diver carries his oxygen supply while exploring a shipwreck in a blue hole.
Cai Yang/Xinhua Press/Corbis
Tourists dive at the Blue Hole off the coast of the Red Sea.