Rejected Choices for the Fifth Discovery

There are lots of competitors for the fifth most important discovery. Heavier than air flight (aka airplanes) have transformed the world. The discovery of electricity makes most of modern society possible. Computers have certainly changed the world. Television has changed the world, mostly for the worse. The internal combustion engine? The dynamo? The magnetron? What about the nuclear bomb? All important, but none of these are my fifth choice.