A loss of trust in currency and a drop in the value would also cause a collapse, but the GDP figure may go up as the value (worth) of the currency is less. (So you'd be moving "more" money around, simply because the value has dropped so more is required to purchase stuff) This is probably why China and Russia are buying up a lot of gold and beginning to trade oil / gas outside of the pretro$ as the dollar is ready to fall off a cliff.