high waves occur most frequently when storm waves drive up against the continental shelves. As waves move away from a storm centre, they change shape, becoming less ragged and taking the form of long smooth swells. such waves can travel very long distances and one such swell, caused by a storm in the Indian Ocean, is known to have traveled 20,000 kilometres (12,400 miles) to Alaska. In 1987, long-distance swells from a storm centre in the Southern Ocean caused extensive flooding and damage in the Maldives.