Cyclones are the most destructive storms in the world. Typhoon Tip, which occurred in the Pacific in 1979, remains the largest and most intense tropical cyclone ever recorded. Cyclones form from clusters of strong thunderstorms and have an eye surrounded by an eyewall that is the calmest area with low pressure and no rain. They are most common in ocean regions like the Pacific, where around 15 occur each year, and can cause major economic damage, like Hurricane Katrina which cost nearly $1 billion.