Six years or so of constantly accelerating house price appreciationcould not go on forever. The exact moment when abubble will burst seems impossible to predict, but burst itdid, at the end of 2006 (see Figure 4). With house pricesnow falling and resets coming on line, subprime delin-quencies began rising steeply (see Figure 5), and the wholestructure simply crumbled. House values quickly fell belowthe amount of the mortgage debt (since there was no significantdownpayment cushion) and, for those homeowners,the embedded option was clearly out of the money. Many ofthese “underwater” loans went into default and foreclosure,and the lower tranches of MBS pools incurred losses, whilethe upper tranches were obviously becoming more risky andhence declining in value.