Filmmaker Tom Fitz describes the first time he found himself sitting, alone, more than 20 meters below Earth’s surface and about 300 meters into an underwater cave: He was waiting in position to film a sequence of divers swimming through a narrow passageway as their lights illuminated the chamber for his new, yet to be named, documentary. “I was suddenly in absolute, complete black,” Fitz says. “The kind of darkness that we rarely experience.”