This year’s floods in Thailand, now threatening central Bangkok, have killed 437 people and done tens of millions of dollars’ worth of damage. But it is far from the worst flood in history. By comparison, the deadliest US flood killed about 2,000 people, when the South Fork dam, upstream from Johnstown, Pa., collapsed on May 31, 1889, after unusually heavy rain. And even that pales beside the destruction wrought by the five deadliest floods in history – all of which took place in China. When did they happen, and just how deadly were they?