We're aboard the Ningaloo Blue, two miles out in the Indian Ocean, about to swim with the world's largest known sharks. Nine years earlier, an amateur biologist discovered that the extremely rare and elusive whale shark regularly comes to this remote part of the northwestern Australia coast shortly after the first full moon in April. Once here, the sharks bask near the surface for about two months and then leave. So far, no scientist has been able to fully explain either their visits or their clockworklike schedules...