Luke Mullins, On Friday April 23, 2010, 5:28 pm EDT

Nearly four years after the real estate market peaked, an alarming number of Americans remain in danger of losing their homes. A non-seasonally adjusted 15 percent of home mortgages were either delinquent or in foreclosure at the end of the fourth quarter of 2009, according to the Mortgage Bankers Association . That's the highest-ever tally in the history of the MBA's National Delinquency Survey.

Mike Larson of Weiss Research points to two key factors behind these high delinquencies. Sharply falling real estate values have put about 21 percent of homeowners underwater, meaning that they owe more on their mortgage than their home is worth. Property owners in this position--which is also known as having negative equity--may find it in their best interest to simply walk away from the home (even, in some cases, when they can afford to make their monthly payments). At the same time, an uncomfortably high national unemployment rate of 9.7 percent means that many Americans won't have the income they need to pay their bills.

[Slide Show: 10 Cities Facing Double-Whammy Defaults.]

Today , some particularly hard-hit markets are in the unenviable position of having both elevated unemployment and high concentrations of negative equity. "Clearly, those are the markets where you are going to see some of the worst metrics on the foreclosure side," Larson says. "You are going to see a lot of people walking away [and] you are going to see a lot of distressed inventory that's being dumped on the market." To pinpoint housing markets that are facing these twin default risks, U.S. News compared negative equity data from Zillow with unemployment figures from Moody's Economy.com. (All data refers to the fourth quarter of 2009.) Based on this data, here is a look at 10 cities that face a double whammy of default risks.

[See How Strategic Defaults Are Reshaping the Economy.]

1. Las Vegas: Speculators and exotic loans pushed home prices in this gambling Mecca dramatically higher during the first half of the previous decade. But after peaking in 2006, the real estate market's crash cleaned out investors and submerged an alarming portion of area homeowners. Through the fourth quarter of 2009, more than 81 percent of single-family home mortgages in Las Vegas were underwater. Meanwhile, the implosion of the housing sector has hammered the local labor market, says Larry Murphy, the president of SalesTraq. When the housing market was sizzling, construction emerged as a key job provider for Las Vegas residents. But as home prices tumbled, the jobs disappeared. "When the housing market goes in the tank, the construction market goes in the tank," Murphy says. "Then you have unemployment and those people can't buy [property] and so it's kind of like a death spiral." The unemployment rate in Las Vegas reached 13 percent in the fourth quarter of last year.

2. Merced, Calif.: California residents looking for alternatives to pricey big cities helped send home prices surging in places like Merced during in the early to middle parts of the last decade. Real estate values in this city of 77,000 residents, which is located east of San Francisco, increased at monster rates before running out of steam in 2006. The proliferation of exotic, adjustable-rate mortgages played a key role in this development, says John Walsh, the president of DataQuick. But the subsequent crash dragged more than 64 percent of area homeowners underwater through the fourth quarter of 2009. And the impact of the real estate bust stretched beyond home prices. "You go to places like Merced and you've got a real significant percentage of the population [that] was involved in either home building, home financing, or home sales," Walsh says. "And all of the sudden all three pieces of those are gone." As a result, Merced's unemployment rate stood at 19 percent through the fourth quarter of 2009.

3. El Centro, Calif.: The same forces that upended Merced's housing and labor markets also hammered the city of El Centro, Walsh says. Residents looking for a cheaper alternative to nearby San Diego moved to El Centro, increasing home prices in this city of 40,000, Walsh says. But when home prices crashed, nearly 57 percent of homeowners found themselves underwater through the fourth quarter of 2009. And without real estate-related industries churning out jobs, the unemployment rate has hit nearly 30 percent.

4. Port St. Lucie, Fla.: The housing market in Port St. Lucie, located on the southeast coast of Florida, experienced one of the most aggressive pricing booms in the state, says Jack McCabe of McCabe Research & Consulting. But the run-up in real estate values wasn't underpinned by growth in population or jobs. "These were markets that were heavily dominated by investor flippers, speculative flippers," McCabe says. "They had no intention of ever occupying the property." When prices crashed, more than 55 percent of single-family homeowners found themselves underwater through the fourth quarter of 2009. And as stagnant sales undercut the housing sector's ability to create jobs, area unemployment reached 14 percent.

5. Fort Myers, Fla.: Over on Florida's west coast, the housing market in Fort Myers experienced a similar phenomenon. An aggressive boom-and-bust cycle has handed negative equity positions to 55 percent of single-family homeowners. And like other housing-boom hotspots, the pain hasn't been limited to real estate values. "We had extremely low unemployment during the boom years because it was all construction jobs," McCabe says. "There was no industry growth and there was no company growth. These were all real estate-related businesses--brokers, title companies, appraisers, and on and on." After the housing euphoria subsided, many employees of real estate-related companies lost their jobs. Unemployment in the Ft. Myers area hit 14 percent in the fourth quarter of 2009.

6. Bend, Ore.: Vacation home buyers, speculative investors, and unique land-use laws worked to drive home prices in Bend sharply higher during the housing boom, says Lester Friedman, president-elect of the Central Oregon Association of Realtors. But as the market petered out, prices headed south in a hurry. "When the market turned, all of a sudden instead of multiple bidders, you've got multiple sellers and very few buyers," Friedman says. Declining real estate values dragged nearly 41 percent of Bend's homeowners underwater. Meanwhile, the housing bust hit the local economy by eroding demand for wood products, an industry that expanded swiftly as real estate values climbed, according to Celia Chen of Moody's Economy.com. Friedman notes that weakness in the tourism sector, which slowed along with the broader economy, has also helped lead to an unemployment rate that topped 14 percent in the fourth quarter of 2009.

7. Ocala, Fla.: The central Florida community of Ocala, which is located north of Orlando, is in the same precarious position as the coastal cities of Port St. Lucie and Fort Myers. Thirty-six percent of homeowners in Ocala are underwater, and area unemployment stood at 14 percent in the fourth quarter of last year. "All throughout Florida--from one coast to the other and in between--the market was overdeveloped and overbuilt," McCabe says. "And that includes the Ocala market."

8. Detroit: A number of cities located outside of the housing-boom hotspots are also facing the twin dangers of high unemployment and negative equity. The erosion of its traditional manufacturing industrial base has helped drive unemployment in the Detroit area to more than 16 percent through the fourth quarter of 2009, Chen says. "And at the same time, there was some very aggressive lending going on during the housing bubble," Chen says. "So many buyers were getting credit who probably shouldn't have gotten credit." High unemployment and exotic home loans have combined to drag nearly 26 percent of area homeowners underwater through the fourth quarter of 2009.

9. Rockford, Ill.: These same forces have worked to land Rockford--a city of 157,000 located in northern Illinois--in a comparable fix, Chen says. Local unemployment hit 16 percent in the fourth quarter of 2009. "The Midwest did go into the recession earlier than the rest of [the country], so the situation has been eroding for a longer period of time," Chen says. At the same time, more than 22 percent of homeowners had negative equity in the final three months of last year.

10. Toledo, Ohio: The housing market in Toledo also faces high unemployment and negative equity. In the fourth quarter of 2009, local unemployment stood at more than 12 percent and roughly 28 percent of homeowners had negative equity. As was the case for Rockford and Detroit, Chen fingered the disappearance of manufacturing jobs and the proliferation of risky mortgages for Toledo's housing headaches.