A cumulative upward movement in the price of an asset that occurs mainly because speculators believe the price will rise still further. Such speculative behaviour can force prices to rise for some period on a path that is eventually realized to be unsustainable; at some point a bubble will burst, but it is hard to predict when this will happen. Historical examples are the South Sea Bubble in 1720, in which the stock price of the companies involved rose quickly before crashing, and the Dutch tulip mania which saw the price of tulip bulbs reach unprecedented heights in 1636–7. More recently, the financial crisis of 2008 has been attributed to the bursting of a bubble in the US housing market.