EVERY YEAR, GREAT white sharks travel over 12,000 miles from South Africa to Australia, charting a nearly perfect straight line across the ocean. And every year, they turn around and travel back. There are no street signs to guide them and, for much of the journey, no stable landmarks by which they can set their course. Currents and water temperatures change. The sun sets at night, the stars disappear during the day. But the sharks carry on.