One in four mortgages today is underwater, and it might get worse. According to Deloitte, nearly half of all homeowners by 2011 will owe more than their homes are worth as property values continue to spiral. These disastrous negative-equity numbers forecast a potential catastrophe for bankers and mortgage services in bringing forth more "walkaways" - people who strategically default on mortgages because the property value has sunk too far below their principal loan obligation.

It's a problem that's already caused up to 26 percent of current foreclosure activity, according to a University of Chicago economist, and could get worse as people see their neighbors skipping out and mailing in keys - often after months of rent-free living. "I'm really concerned" about walkaways, says Luigi Zingales, an economics professor studying the foreclosure crisis at UofC's Booth School of Business. "This can spread like wildfire."

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