Natural disaster - this is a huge event, like tornado, hurricane, or Earthquakes.

Natural disasters:
The Huang Ho (Yellow River) in China floods particularly often. The Great Flood of 1931 caused between 800,000 and 4,000,000 deaths.
The Great Flood of 1993 was one of the most costly floods in US history.
The 1998 Yangtze River Floods, also in China, left 14 million people homeless.
The 2000 Mozambique flood covered much of the country for three weeks, resulting in thousands of deaths, and leaving the country devastated for years afterward