Project Stormfury was an experimental program run by the US government from 1962 to 1983. The project's goal was to weaken hurricanes by seeding clouds in the eyewall of the storm with silver iodide. It was thought the silver iodide would cause supercooled water in the storm to freeze, disrupting the inner structure of the hurricane.
In the mid-80s, researchers discovered that hurricanes contained too little supercooled water for seeding to be effective. They further observed that seeded and unseeded hurricanes often undergo the same structural changes, which cast doubts on the program's results. This finding eventually led to the cancellation of Project Stormfury. Read more about Project Stormfury here(1).
Graphic credit: aoml.noaa.gov(2)