Extend the line of the west coast of Tasmania northwards and you strike the mainland of Australia at Cape Otway, on the Victorian coast. Midway along this line lies King Island, a substantial piece of land. It rises from the submarine shelf that connects Tasmania with mainland Australia, towards its outer, western edge. There is blue water round the island, hundreds of feet deep. But westwards, off this submarine shelf, the sea bottom slides down to ocean blackness. If you were in a plane taking off from King Island on a really clear day, you would be able to see…