Kenji Yamaguchi is National Geographic's in-house camera hacker who mods lenses, builds "camera traps," and customizes gear of all kinds for the magazine's shooters in the field.
From National Geographic:
One of his favorite experiences occurred a few hours away from his native Tokyo when he helped underwater photographer David Doubilet with a month-long assignment in Suruga Bay, Japan. His main challenge was getting two scenes on one frame (a double exposure). Kenji's role was to load the film and help capture the images in a single, seamless photo (below).
The photographic team used one piece of film to create two different images. First they exposed the lower half of the negative underwater, then expose the upper half above water several days later.