By Jane Wardell SYDNEY (Reuters) - The southern Indian Ocean, where investigators suspect missing Malaysia Airlines Flight MH370 may have come down, is one place where a commercial airliner can crash without a ship spotting it, a radar plotting it or even a satellite picking it up. Even Australia, which has island territories in the Indian Ocean and sends rescue planes to pluck stricken yachtsmen from the cold, mountainous seas in the south from time to time, has no radar coverage much beyond its Indian Ocean coast. "In most of Western Australia and almost all of the Indian Ocean, there is almost no radar coverage," an Australian civil aviation authority source said, requesting anonymity as he was not authorized to speak on the record. "If anything is more than 100 kilometers offshore, you don't see it." The Indian Ocean, the world's third largest, has an average depth of more than 12,000 feet, or two miles.