One of the mysteries of nature is animal communication. Creatures of the sea move through their liquid element without any audible sound. Yet they are able to sense the silent passing or attack of others of their kind. This is because bodies moving in water create a pressure wave, something like the puff of wind produced by a passing automobile. A narrow sensor system running along each side of the shark, from eye to tail, receives and interprets these pressure waves. One might think it impossible to distinguish odors in water. Yet sharks, surprisingly, are able to follow a scent across miles of ocean and arrive at its exact source. A diver who spears a fish and attaches its bloody body to his belt, and then goes on with the hunt, becomes a natural prey to the shark. They can follow a scent exactly like dogs of a hunting pack. It is no wonder that the Greeks gave them the name of "hounds of the sea. \