The 9/11 attacks are the most costly man-made catastrophe, at least if you look at them from the perspective of the insurance industry. The attacks against the World Trade Center, the Pentagon and other buildings in the US cost more than 25 billion dollars in damages.
Mostly it’s Mother Nature who causes havoc on an equal or even bigger scale. Then again, some argue that while this type of disasters isn’t directly caused by mankind, the human impact on nature increasingly makes itself felt in the form of extreme weather conditions.
Even if you want to blame nature alone, events like Hurricane Katrina, show that the effects of such catastrophes can be exacerbated by lack of preparation and inappropriate response to natural disaster. Katrina was the costliest natural disaster in recent history by far.