TEOTWAWKI - The End of the World as We Know It - is the the ultimate SHTF. This is SHTF for good, a theoretical situation where things get so bad that there's no going back to the old way of life and those who survive will be starting from scratch, reduced to fighting for their lives and eking out an existence any way possible.
TEOTWAWKI scenarios are a popular theme in survival fiction, both in books and movies. Every civilization comes to an end eventually, and ours will probably be no different. But TEOTWAWK is not what most of us are preparing for, because when such an event does happen, most people won't be left alive to try to survive it. Cormac McCarthy's prize-winning novel, The Road paints a realistically grim picture of what life would be like post-TEOTWAWKI.