This thread is about two things: identifying large scale potential catastrophes and then making realistic plans about what to do in the event, likely or unlikely, that it actually occurs. Catastrophes that effect anything smaller than a large city or larger than our solar system need not apply (but feel free to start a thread about them).
I'll begin:
YELLOWSTONE COULD KILL US ALL!!!
Under Yellowstone National Park is one of the largest
calderas known to man measuring at 34 miles (55 km) by 45 miles (72 km). While we've known about the caldera for over 30 years, it was only classified as a "supervolcano" recently. While Wyoming does not fall on a
fault line, it is a known
hot spot, or an area of land that features volcanic activity directly from the mantle, straight up.
According to people in white lab coats, over the past 17 million years this particular caldera has erupted many times, covering Washington, Oregon, California, Nevada, and Idaho in lava, and covering much of North America in ash. These eruptions are roughly 2,500 times the size of the eruption seen at Mt. St. Helens in 1980. As far as we know, the most recent eruption happened about 640,000 years ago and is responsible for the largest volcanic activity of the Pleistocene.
A full eruption of this caldera would almost certainly result in the deaths of millions initially, and could kill billions. The following image shows a theoretical map of
total destruction:
WHAT SHOULD I DO?!
If you're in the zone illustrated above: get the fuck out asap. Plan ahead by making sure your car always has gas and that you have a clear route away from the theoretical center of the eruption. There is no way to know how this will erupt, so your best bet is to simply drive in the opposite direction of Yellowstone. Make sure that you have gas masks to prevent inhalation of ash, and you have plenty of non-perishable food, extra fuel, and a satellite capable phone. Get as far as you possibly can and call for help as soon as possible.
If you're not in the zone illustrated above: this is still likely to have a serious effect on you. Have plenty of non-perishable food items and water filters. If you're in North America, don't expect to see the sun for a bit. Europe is also likely to see quite a bit of ash.
Long term: this will take years to get out of the air and the agriculture industry will probably take decades to recuperate, should there be infrastructure left when the planet starts getting back to normal. Try to plant as many gardens as possible. Clean up ash from local parks and try planting there.
Sources:
http://en.wikipedia.org/wiki/Yellowstone_Caldera
http://people.uwec.edu/ERICKSKM/hazards.html
www.armageddononline.org