Once it starts to go over the cliff, it's gone."

In other parts of the country, the foreclosure wave was barely a ripple — at least until it started swamping major banks that had invested heavily in mortgages. Banking giant Wachovia Corp., for example, was hammered after California and Florida customers of one mortgage firm it bought began defaulting at high rates. The risks of such lending were spread so broadly among financial institutions that, when the loans went bad, it drove the national credit crisis, says Christopher Mayer, who studies real estate at Columbia Business School.

The Obama administration on Wednesday detailed a $75 billion plan to keep more homeowners from slipping into foreclosure by helping them refinance loans or reduce their monthly payments. But that effort could face political challenges because most of the foreclosure problem has been so concentrated in a few areas, says Brookings Institution researcher Alan Mallach.