Up to four million people died in the deadliest natural disaster of the 20th century, killing as many as 15 times the toll of the 2004 Indonesian tsunami. It reshaped a nation's geography and forced tens of millions to migrate. Yet there is a good chance you have never heard of the 1931 China floods.

This episode unpacks how a catastrophe of this scale could happen and how it was almost completely erased from history. We trace the perfect storm of meteorological factors, the biological aftermath that killed far more than drowning, and the geopolitical timing that swallowed the memory of millions.

• A three-year drought baked the soil hydrophobic, a harsh winter locked up ice, then July 1931 brought nine cyclones and over 600 millimeters of rain at four stations.
• Floodwaters covered roughly 180,000 square kilometers, an area ...