National Geographic : 1974 Oct
marine food chain, from the smallest single celled algae on up to fishes and man.* We concentrated primarily on visual obser vations of these remarkable animals. To avoid getting lost in the endless blue of the deep ocean-which dropped nearly 500 fath oms below us-we dived within a grid of lines hanging from surface floats in a prearranged geometric array. Each diver-researcher was free to concen trate on the tiny animals performing six inch es from his nose because he was tethered to a safety man suspended at a given depth with in the grid. When necessary, the safety man disentangled the lines of the two or three re search divers tied to him. He also watched each man's depth and elapsed underwater time, and kept a lookout for sharks. On many dives, large, curious, and poten tially dangerous fishes would suddenly glide into view. Most of these were six- to eight foot-long gray oceanic sharks, occasionally *See "Algae: the Life-givers," by Paul A. Zahl, NA TIONAL GEOGRAPHIC, March 1974.