There are several different events that could result in circumstances that would require extreme survival measures, such as a Nuclear War or X Class Solar Flares.
These events would result in major destruction and death, as well as total social breakdown.
Could you survive if you were left totally on your own?
Below is a five part series called the 'Apocalypse Man' presented by The History Channel during its "Armageddon Week". This series features an Afghanistan and Iraq War veteran Rudy Reyes who demonstrates how you could possibly survive a catastrophe of mass destruction.
Reyes walks you through finding food, water, heat, and basic shelter; as well as how to make fire from steel wool, how to find safe houses in abandoned cities, and how to make a short wave radio out of things on hand.