Colombo did manage to renegotiate a new fixed interest rate loan with his bank, and now believes he'll be OK - but the future is less certain for the rest of us.

In the months ahead, millions of other adjustable-rate mortgages like Colombo's will reset, giving them a higher interest rate as required by the loan agreements and leaving many homeowners unable to make their payments. Soaring mortgage default rates this year already have shaken major financial institutions and the fallout from more of them, some experts say, could spread from those already battered banks into the general economy.

The worst-case scenario is anyone's guess, but some believe it could become very bad.

"We haven't faced a downturn like this since the Depression," said Bill Gross, chief investment officer of PIMCO, the world's biggest bond fund. He's not suggesting anything like those terrible times - but, as an expert on the global credit crisis, he speaks with authority.

"Its effect on consumption, its effect on future lending attitudes, could bring us close to the zero line in terms of economic growth," he said. "It does keep me up at night."

Some 2 million homeowners hold $600 billion of subprime adjustable-rate mortgage loans, known as ARMs, that are due to reset at higher amounts during the next eight months. Subprime loans are those made to people with poor credit. Not all these mortgages are in trouble, but homeowners who default or fall behind on payments could cause an economic shock of a type never seen before.

Some of the nation's leading economic minds lay out a scenario that is frightening. Not only would the next wave of the mortgage crisis force people out of their homes, it might also spiral throughout the economy.

The already severe housing slump would be exacerbated by even more empty homes on the market, causing prices to plunge by up to 40 percent in once-hot real estate spots such as California, Nevada and Florida. Builders like Chicago's Neumann Homes, which filed for bankruptcy protection this month, could go under. The top 10 global banks, which repackage loans into exotic securities such as collateralized debt obligations, or CDOs, could suffer far greater write-offs than the $75 billion already taken this year.

Massive job losses would curtail consumer spending that makes up two-thirds of the economy. The Labor Department estimates almost 100,000 financial services jobs related to credit and lending in the U.S. have already been lost, from local bank loan officers to traders dealing in mortgage-backed securities. Thousands of Americans who work in the housing industry could find themselves on the dole. And there's no telling how that would affect car dealers, retailers and others dependent on consumer paychecks.

Based on historical models, zero growth in the U.S. gross domestic product would take the current unemployment rate to 6.4 percent. That would wipe out about 3 million jobs from the economy, according to the Washington-based Economic Policy Institute.

By comparison, in the last big downturn between 2001-03 some 2 million jobs were lost, according to the Labor Department. The dot-com bust early this decade decimated the technology sector, while the Sept. 11, 2001, terror attacks hurt the transportation and allied industries. Economists said the country was officially in recession from March to November of 2001, but the aftermath stretched to 2003.

There is increasing evidence that another downturn has begun.

Borrowers who took out loans in the first six months of this year are already falling behind on their payments faster than those who took out loans in 2006, according to a report from Arlington, Va.-based investment bank Friedman, Billings Ramsey. That's making it even harder for would-be buyers to get new mortgages - a frightening prospect for home builders with projects going begging on the market, and for homeowners desperate to unload property to avoid defaulting on their loans.

Meanwhile, the number of U.S. homes in foreclosure is expected to keep soaring after more than doubling during the third quarter from a year earlier, to 446,726 homes nationwide, according to Irvine, Calif.-based RealtyTrac Inc. That's one foreclosure filing for every 196 households in the nation, a 34 percent jump from just three months earlier.

Such data suggests more Americans could lose their homes than ever before, and those in peril are people who never thought they'd welsh on a mortgage payment. They come from a broad swath - teachers, pharmacists, and civil servants who were lured by enticing mortgage terms.

Some homebuyers gambled on interest-only loans. The mortgages, which allowed buyers to pay just interest at a low rate for two years, were too good to pass up. But with that initial term now expiring, many homeowners find they can't make the payments. The hopes that went along with those mortgages - that they'd be able to refinance because the equity in their homes would appreciate - have been dashed as home prices skidded across the country.

"It's been said a lot of people have been using their homes as ATM machines," said Thomas Lawler, a former official at mortgage lender Fannie Mae who is now a private housing and finance consultant. "The risk has a lot of tentacles."

This example illustrates the distress many homeowners are in or will find themselves in: A subprime adjustable-rate mortgage on a $400,000 home could have payments of about $2,200 a month, with borrowers paying 6.5 percent, interest only. When the teaser period expires, that payment becomes $4,000, with the homeowner paying 12 percent and now having to come up with principal as well as interest.